limit github action to chapel-lang repo (#26680)

This adds a restriction to the `publish-docs` github action so that it
only runs on `chapel-lang/chapel` and not on forks or mirrors of the
repository, where it would always fail due to missing secrets.

Tested against my own fork with and without matching repository names. 

Trivial change to solve an annoyance in an action that isn't actually
doing anything yet. Not reviewed.

.github/workflows/CI.yml

@@ -192,7 +192,7 @@ jobs:
           done
 
   publish-docs:
-    if: github.event_name == 'push'
+    if: github.event_name == 'push' && github.repository == 'chapel-lang/chapel'
     runs-on: ubuntu-latest
     needs: make_doc
     steps: