Root cause: compiler_check=mtime (default) invalidates the entire
cache on every run because pixi installs fresh compiler binaries with
new mtimes each time. The CI logs confirm 0/2579 cache hits (0.00%)
despite successfully restoring a 225MB cache.
Fix: set CCACHE_COMPILERCHECK=content to hash compiler binary content
instead of mtime. Same compiler version = same content = cache hits.
Also fix cache save strategy: use github.run_id for save keys so every
build persists its ccache objects. Previously date-based keys with
cache-hit guard meant the cache was only saved once per day — all
subsequent compilations were thrown away.
Changes:
- Add CCACHE_COMPILERCHECK=content to build.yml, release.yml, build.sh
- Use github.run_id in cache save keys (unique per run = always saves)
- Remove date-based cache key computation step
- Remove cache-hit != true guard on save step
- Add ccache-build-main- fallback to release workflow restore-keys
