Fix CI check to disallow template testing on forked repositories

#2222 used the wrong syntax / check. This PR corrects that.

.github/workflows/ci.yml

@@ -82,7 +82,7 @@ jobs:
   update-templates-to-examples:
     # this doesn't work on forked repositories (i.e. outside contributors)
     # so we disable template updates for those PRs / branches
-    if: github.repository == 'zenml-io/zenml'
+    if: github.event.pull_request.head.repo.full_name == "zenml-io/zenml"
     uses: ./.github/workflows/update-templates-to-examples.yml
     with:
       python-version: "3.8"