ci: make ccache effective and observable, shallow checkout, add cache…

[MrLi000001]

Measured on the self-hosted runners, the Build step of the same workflow ranges from 2.7 min (warm ccache) to 37.5 min (stale ccache), and a full-history checkout took ~12 min on cache-cold runners.

• Set ccache --max-size=30G (default ~10G is tight for -O3 -g builds) and print ccache -s before/after the build so hit rates are visible in every run log (if: always()).
• fetch-depth: 0 -> 1; CMake only runs 'git log -1' for commit info (CMakeLists.txt:163-174).

Note: the cache warmer was split out of this PR per review (Stardust0831 / Critsium-xy) - /tmp/ccache persistence across the dynamic runner pool is still unverified, and the warmer's CUDA build flags were out of sync. It will return as its own PR (with timeout-minutes and matching build flags) once the persistence question is answered.

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[Stardust0831]

The shallow checkout and ccache statistics are useful, but I suggest splitting out the cache warmer. It is not yet clear whether /tmp/ccache persists across jobs and nodes in the dynamic runner pool, so one warmer may only warm a single node while consuming an additional GPU. It also builds the default seven CUDA architectures, while #7753 uses sm_70, so those CUDA cache entries will not be effectively reused.

[Critsium-xy]

A few concerns on the cache_warmer.yml half (the test.yml changes look fine to me — I checked that fetch-depth: 1 is safe, since the only git-history consumer is git log -1 in CMakeLists.txt:163-174):

1. The configure flags are copy-pasted and already out of sync. The header comment says to keep them aligned with test.yml / cuda.yml, but as of this commit they already differ: -Werror=dev is missing, and the parallelism is -j8 / -j4 / -j $(nproc) across the three files. A comment can't hold three copies of a build recipe together. Suggest extracting "install toolchain + configure + build" into a workflow_call reusable workflow (or a composite action) that all three call with parameters, so there is one source of truth.

2. /tmp/ccache persistence across the runner pool is unverified. /tmp/ccache is a host path, so if the X64 / gpu pools are multi-node or ephemeral, the warmer only primes whichever node it happens to land on, and every other node still pays the cold build. This is the premise the whole warmer rests on — worth confirming first (e.g. print hostname alongside ccache -s over a few runs and check whether the cache size accumulates on the same host).

3. Neither warmer job sets timeout-minutes. The default is 6 hours. A hung build would hold a self-hosted runner — including a scarce GPU one — for that long and starve PR CI. Please add something like timeout-minutes: 60.

Given 2 and 3, I'd suggest splitting as @Stardust0831 proposed: land the test.yml part now, and bring the warmer back as its own PR once the cache-persistence question is answered.