ci: persist ccache across builds

[greatgitsby]

CI cached whole build trees (*/*-src, */build, ~2 GB per platform with #101) under one key, and ccache's dir wasn't in it at all. Two problems: the git checkout --force every build.sh does rewrites mtimes on a restored tree, so everything recompiles anyway; and the entries are big enough to evict each other (repo cache is 10 GB), so a busy PR makes every other branch cold.

Now: cache only .ccache, keyed per run so it's always saved fresh (~300 MB), plus .uv-cache and the two download-only packages. Builds start from clean trees; ccache absorbs the compile. Also export the ccache compiler in ffmpeg (x264/zlib/meson) and cppcheck (make), which were bypassing it.

build job, master tree, empty commits between stages:

	cold	+ccache in old entry	+export fix	old entry evicted (cold)	per-run ccache
linux-x86_64	10m36s	5m20s	3m44s	10m09s	4m08s
linux-arm64	6m27s	3m45s	3m06s	7m23s	3m40s
macos	6m17s	3m58s	3m10s	9m28s	4m34s

cache entries: 20–31 MB ccache per platform + 148 MB downloads, vs ~2 GB before.

[greatgitsby]

opensuse mirror is flaky (syncing now?)

separate issue.