Use CI_MERGE_REQUEST_LABELS env var in undercoverage

.gitlab/ci/rails.gitlab-ci.yml

@@ -594,8 +594,7 @@ rspec:undercoverage:
   stage: post-test
   needs: ["rspec:coverage"]
   script:
-    - apt install -y jq
-    - if [[ $(curl "${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/merge_requests/${CI_MERGE_REQUEST_IID}" | jq ".labels" | grep "pipeline:skip-undercoverage") ]]; then
+    - if [[ ",${CI_MERGE_REQUEST_LABELS}," == *",pipeline:skip-undercoverage,"* ]]; then
         echo "The 'pipeline:skip-undercoverage' label is set on the MR, exiting early.";
         exit 0;
       else