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.