gitlab-ci: Use ccache

Meson picks it up automatically.

Based on:

* https://gitlab.freedesktop.org/mesa/mesa/merge_requests/240
* https://gould.cx/ted/blog/2017/06/10/ccache-for-Gitlab-CI/
* https://stackoverflow.com/questions/53659419/ccache-no-hits-in-gitlab-ci

v2 based on the corresponding Mesa change:
* Quote CCACHE_(BASE)DIR environment variables.
* Clear ccache stats in before_script.
* Move cache stanza to the build-and-test job, the cache isn't used in
  the docker-image job.

Reviewed-by: Adam Jackson <ajax@redhat.com> # v1
(Cherry picked from commit b577df77)