Make ESLint job fail when there are findings

Just a demo MR to see that the ESLint job fails while we still have errors.

.gitlab-ci.yml

@@ -35,3 +35,10 @@ variables:
   _PHPUNIT_EXTRA: --verbose
   # Convenient, and we have no secrets.
   _SHOW_ENVIRONMENT_VARIABLES: 1
+
+# Override values for the default eslint job.
+eslint:
+  # We have set a baseline where ESLint passes. If we really need to get a pass,
+  # let's make it explicit.
+  # See https://eslint.org/docs/latest/use/configure/rules#disabling-rules
+  allow_failure: false