CI: Add EIGEN_CI_SCHEDULE_SCOPE=nightly for cache-warming Linux builds

Select the seven existing Linux full-build jobs that back the merge-request
smoketest and default affected-tests tiers when a scheduled pipeline sets
EIGEN_CI_SCHEDULE_SCOPE=nightly:

  - build:linux:cross:x86-64:gcc-10:default
  - build:linux:cross:x86-64:gcc-10:avx2
  - build:linux:cross:x86-64:clang-14:default
  - build:linux:cross:aarch64:gcc-10:default
  - build:linux:cross:aarch64:clang-14:default
  - build:linux:riscv64:gcc-15:default
  - build:linux:riscv64:clang-21:default

Also use the pre-baked ubuntu-24.04-amd64-smoketest-build image (with
EIGEN_CI_SKIP_APT=true) on the x86-64 and aarch64 gcc-10/clang-14 parent build
jobs so full builds, :affected, and :smoketest share the exact same compiler
binary digest in sccache/ccache and skip redundant per-job apt installs.