Fix undercoverage not existing in older branches

We deliberately reset to the MR source branch for comparison.
However the undercoverage script, and gem might not be present for older
MR source branches. So, as a transitionary measure we skip undercoverage
if the script file is not present.

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

@@ -537,7 +537,9 @@ rspec:undercoverage:
       else
         echo "Using \$CI_COMMIT_SHA ($CI_COMMIT_SHA) for this non-merge result pipeline.";
       fi;
-    - run_timed_command "scripts/undercoverage"
+    - if [ -f scripts/undercoverage ]; then
+        run_timed_command "scripts/undercoverage";
+      fi;
 
 rspec:feature-flags:
   extends: