Fix broken undercoverage job

As we switch back to the source branch, we need to run bundle install
again to install missing gems

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

@@ -534,6 +534,7 @@ rspec:undercoverage:
     - if [ -n "$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA" ]; then
         echo "Checking out \$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA ($CI_MERGE_REQUEST_SOURCE_BRANCH_SHA) instead of \$CI_COMMIT_SHA (merge result commit $CI_COMMIT_SHA) so we can use $CI_MERGE_REQUEST_DIFF_BASE_SHA for undercoverage in this merged result pipeline";
         git checkout -f ${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA};
+        bundle_install_script;
       else
         echo "Using \$CI_COMMIT_SHA ($CI_COMMIT_SHA) for this non-merge result pipeline.";
       fi;