Override GIT_STRATEGY in .gitlab-ci.yml

This adds a hard-coded 'GIT_STRATEGY: clone' to .gitlab-ci.yml.
Without this, misconfigured forks of gitlab-org/gitlab can use the
'fetch' strategy which is bad for the pack-objects cache hit ratio.
Without a good hit ratio, such misconfigured forks cause CPU
saturation events on the Gitaly server that hosts them.

See https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1584
for more information.

.gitlab-ci.yml

@@ -65,6 +65,8 @@ variables:
   # we override the max_old_space_size to prevent OOM errors
   NODE_OPTIONS: --max_old_space_size=3584
   GIT_DEPTH: "20"
+  # 'GIT_STRATEGY: clone' optimizes the pack-objects cache hit ratio
+  GIT_STRATEGY: "clone"
   GIT_SUBMODULE_STRATEGY: "none"
   GET_SOURCES_ATTEMPTS: "3"
   DEBIAN_VERSION: "bullseye"