ci: share ccache directory between branches

utils/tests/gitlab-ci.yml

@@ -41,7 +41,15 @@ stages:
     - ./waf
     - if [ "$MODE" != "debug" ]; then ./test.py --nowaf; fi
   cache:
-    key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
+    # Use separate key for each (debug/optimized/release) jobs because
+    # they run in parallel and will otherwise overwrite each other
+    # cache when they upload the cache archive at the end of the job,
+    # resulting in only the cache for the last finished configuration
+    # being stored.
+    #
+    # Do not distinguish between branches though to avoid
+    # recompilation of all the files when a new branch is created.
+    key: "$CI_JOB_NAME"
     paths:
       - $CCACHE_BASEDIR_VALUE/
   timeout: 9h