arm64: make sure the cache size stays in check

[lazka]

since we disable caching now we also no longer prune the cache

[jeremyd2019]

What does the mtime represent? Some tools set the mtime to the last modified header when downloading, so I don't think it would be very meaningful in this context.

[lazka]

right, I tested and pacman keeps the original mtime. So we should use atime instead.

[jeremyd2019]

I'm not sure atime works either ☹️ . The expression paccache uses is find "$PWD" -name '*.pkg.tar*.sig' -prune -o \( -name '*.pkg.tar*' -printf '%A@ %T@ %p\n' \) and when I run that I find a different atime (first column) every run, so I think atime gets updated by the check 🤦

[jeremyd2019]

I think I'd either just do what setup-msys2 does (-rfuk0, -rfk1), or just ignore it and see what happens.