ci(gitlab-ci): improve pre-commit caching

* with the previous setting the stored cache would grow
  and grow as long as the job name remained the same

Author: dafyddj

.gitlab-ci.yml

@@ -81,7 +81,10 @@ pre-commit:
   variables:
     PRE_COMMIT_HOME: '${CI_PROJECT_DIR}/.cache/pre-commit'
   cache:
-    key: '${CI_JOB_NAME}'
+    key:
+      files:
+        - .pre-commit-config.yaml
+      prefix: pre-commit
     paths:
       - '${PRE_COMMIT_HOME}'
   script: