ci: Run git gc before creating Git cache tarball

Since the job which creates the cache tarball starts from the previous
cache, the cache kept accumulating cruft and growing bigger.

This cuts the size of the tarball in half (from almost 600M to under
300M), which can translate to significant time savings when downloading
it on some runners.

v2:
* Use git gc --aggressive (Eric Anholt)

Reviewed-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7920>

.gitlab-ci.yml

@@ -479,6 +479,8 @@ make git archive:
   tags:
     - packet.net
   script:
+    # Compactify the .git directory
+    - git gc --aggressive
     # compress the current folder
     - tar -cvzf ../$CI_PROJECT_NAME.tar.gz .