gitlab-ci: Set GIT_STRATEGY to none for the dummy job

It doesn't need anything from the Git repository.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3474>

.gitlab-ci.yml

@@ -65,6 +65,8 @@ success:
   except:
     changes:
       *paths
+  variables:
+    GIT_STRATEGY: none
   script:
     - echo "Dummy job to make sure every merge request pipeline runs at least one job"