ci: sanity job doesn't need the Git tree

This should fix the sanity job taking >= 1 minute in some cases.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7904>

.gitlab-ci.yml

@@ -489,6 +489,8 @@ sanity:
     - if: *is-forked-branch
       when: manual
     # Other cases default to never
+  variables:
+    GIT_STRATEGY: none
   script:
     # ci-fairy check-commits --junit-xml=check-commits.xml
     # ci-fairy check-merge-request only works in pre-merge pipelines for MRs