CI: Address MR review feedback on sccache overrides and multi-arch caching

Address review comments from @rmlarsen1 on MR !3171:
- Add sccache directory and size override fallback chain (SCCACHE_DIR, EIGEN_CI_SCCACHE_DIR) to respect runner-level overrides while capping default project-local cache at 4G.
- Print notice on download_file failure when curl, wget, and python3 are all unavailable.
- Guard SCCACHE_GCS_CREDENTIALS_URL export in build scripts when credential server port is empty.
- Key compiler cache binary directories by architecture (.sccache-bin/${arch}, .ccache-bin/${arch}) so multi-arch runners share linux-ccache-bin without thrashing.

ci/scripts/build.linux.script.sh

diff --git a/ci/scripts/build.linux.script.sh b/ci/scripts/build.linux.script.sh
index 04fffae..b0572db 100755
--- a/ci/scripts/build.linux.script.sh
+++ b/ci/scripts/build.linux.script.sh
@@ -33,8 +33,8 @@
   . "${rootdir}/ci/scripts/install_compiler_cache.sh"
 
   if [[ "${EIGEN_CI_SCCACHE:-on}" != "off" && -n "${sccache_bin}" ]]; then
-    export SCCACHE_DIR="${CI_PROJECT_DIR}/.sccache"
-    export SCCACHE_CACHE_SIZE="${CCACHE_MAXSIZE:-4G}"
+    export SCCACHE_DIR="${SCCACHE_DIR:-${EIGEN_CI_SCCACHE_DIR:-${CI_PROJECT_DIR}/.sccache}}"
+    export SCCACHE_CACHE_SIZE="${SCCACHE_CACHE_SIZE:-${EIGEN_CI_SCCACHE_CACHE_SIZE:-4G}}"
     export SCCACHE_BASEDIRS="${rootdir}"
     export SCCACHE_SERVER_PORT="$((4226 + ${CI_CONCURRENT_ID:-0}))"
 
@@ -78,6 +78,7 @@
       set -x
 
       # Wait up to 2 seconds for local credential server to be ready
+      cred_port=""
       for _ in {1..40}; do
         if [[ -s "${cred_port_file}" ]]; then
           cred_port=$(cat "${cred_port_file}")
@@ -85,7 +86,16 @@
         fi
         sleep 0.05
       done
-      export SCCACHE_GCS_CREDENTIALS_URL="http://127.0.0.1:${cred_port}/token/${GCS_URL_SECRET}"
+      if [[ -n "${cred_port:-}" ]]; then
+        export SCCACHE_GCS_CREDENTIALS_URL="http://127.0.0.1:${cred_port}/token/${GCS_URL_SECRET}"
+      else
+        echo "Notice: Local credential server failed to bind or respond; skipping GCS remote cache." >&2
+        unset SCCACHE_GCS_BUCKET
+        unset SCCACHE_GCS_RW_MODE
+        unset SCCACHE_MULTILEVEL_CHAIN
+        [[ -n "${sccache_cred_server_pid}" ]] && kill "${sccache_cred_server_pid}" 2>/dev/null || true
+        sccache_cred_server_pid=""
+      fi
       rm -f "${cred_port_file}"
       unset GCS_URL_SECRET
     fi

ci/scripts/build.windows.script.ps1

diff --git a/ci/scripts/build.windows.script.ps1 b/ci/scripts/build.windows.script.ps1
index bac52b6..9fb4a05 100644
--- a/ci/scripts/build.windows.script.ps1
+++ b/ci/scripts/build.windows.script.ps1
@@ -85,8 +85,8 @@
 
   # 1. Try starting sccache server if available
   if ($sccache_exe) {
-    $env:SCCACHE_DIR = Join-Path $env:CI_PROJECT_DIR ".sccache"
-    $env:SCCACHE_CACHE_SIZE = if ($env:EIGEN_CI_CCACHE_MAXSIZE) { $env:EIGEN_CI_CCACHE_MAXSIZE } else { "4G" }
+    $env:SCCACHE_DIR = if ($env:SCCACHE_DIR) { $env:SCCACHE_DIR } elseif ($env:EIGEN_CI_SCCACHE_DIR) { $env:EIGEN_CI_SCCACHE_DIR } else { Join-Path $env:CI_PROJECT_DIR ".sccache" }
+    $env:SCCACHE_CACHE_SIZE = if ($env:SCCACHE_CACHE_SIZE) { $env:SCCACHE_CACHE_SIZE } elseif ($env:EIGEN_CI_SCCACHE_CACHE_SIZE) { $env:EIGEN_CI_SCCACHE_CACHE_SIZE } else { "4G" }
     # Rewrite paths relative to rootdir for cross-runner / cross-directory cache hits.
     $env:SCCACHE_BASEDIRS = $rootdir
     # Isolate daemon port per runner slot to avoid port collisions and process cross-kill.
@@ -151,7 +151,19 @@
         }
         Start-Sleep -Milliseconds 50
       }
-      $env:SCCACHE_GCS_CREDENTIALS_URL = "http://127.0.0.1:$cred_port/token/$url_secret"
+      if ($cred_port) {
+        $env:SCCACHE_GCS_CREDENTIALS_URL = "http://127.0.0.1:$cred_port/token/$url_secret"
+      } else {
+        Write-Warning "Local credential server failed to bind or respond; skipping GCS remote cache."
+        Remove-Item Env:SCCACHE_GCS_BUCKET -ErrorAction SilentlyContinue
+        Remove-Item Env:SCCACHE_GCS_RW_MODE -ErrorAction SilentlyContinue
+        Remove-Item Env:SCCACHE_MULTILEVEL_CHAIN -ErrorAction SilentlyContinue
+        if ($cred_server_job) {
+          Stop-Job $cred_server_job -ErrorAction SilentlyContinue
+          Remove-Job $cred_server_job -Force -ErrorAction SilentlyContinue
+          $cred_server_job = $null
+        }
+      }
     }
 
     & $sccache_exe --start-server | Out-Null

ci/scripts/install_compiler_cache.sh

diff --git a/ci/scripts/install_compiler_cache.sh b/ci/scripts/install_compiler_cache.sh
index cc16828..d3cc403 100755
--- a/ci/scripts/install_compiler_cache.sh
+++ b/ci/scripts/install_compiler_cache.sh
@@ -21,6 +21,7 @@
   if command -v python3 >/dev/null 2>&1; then
     python3 -c "import sys, urllib.request; urllib.request.urlretrieve(sys.argv[1], sys.argv[2])" "${url}" "${dest}" && return 0
   fi
+  echo "Notice: Neither curl, wget, nor python3 is available to download $(basename "${dest}")." >&2
   return 1
 }
 
@@ -29,7 +30,7 @@
 
 if [[ -z "${sccache_bin}" ]]; then
   sccache_ver="0.18.0"
-  sccache_bindir="${root_dir}/.sccache-bin"
+  sccache_bindir="${root_dir}/.sccache-bin/${arch}"
   sccache_tar_sha256=""
   sccache_bin_sha256=""
 
@@ -78,7 +79,7 @@
 
 if [[ -z "${ccache_bin}" ]]; then
   ccache_ver="4.13.6"
-  ccache_bindir="${root_dir}/.ccache-bin"
+  ccache_bindir="${root_dir}/.ccache-bin/${arch}"
   ccache_tar_sha256=""
   ccache_bin_sha256=""