gitlab-ci: Directly use host-mapped directory for ccache

gitlab-ci: Directly use host-mapped directory for ccache

Use hardcoded /cache/mesa/ccache for the cache, so it will be shared by
all jobs of all Mesa projects running on the same runner host. This
should increase the hit rate and decrease the worst case storage used.

Further benefits of directly using a host-mapped directory:

* Saves up to ~1 minute per job for restoring and saving the cache
  contents via the GitLab CI cache mechanism
* Cache contents generated by failed jobs are no longer lost
* Jobs running in parallel on the same runner host can get hits from
  each other

Also enable compression, so the default maximum cache size of 5G might
be sufficient.

v2:
* Move CCACHE_DIR variable to the .build-linux template

Suggested-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net> # v1