Correct context key name in "Sync Labels" workflow

Incorrect context key name resulted in impossible to satisfy conditional, meaning the dry run determination code was
solely dependent on the check for whether the workflow was triggered from the default branch name.

Author: per1234

Diff for: .github/workflows/sync-labels.yml

@@ -92,7 +92,7 @@ jobs:
       - name: Determine whether to dry run
         id: dry-run
         if: >
-          github.event == 'pull_request' ||
+          github.event_name == 'pull_request' ||
           github.ref != format('refs/heads/{0}', github.event.repository.default_branch)
         run: |
           # Use of this flag in the github-label-sync command will cause it to only check the validity of the