fix(ci): skip release draft update on pull_request events

The release-drafter action fails with 'Validation Failed: target_commitish'
when triggered by pull_request events because it tries to set the PR merge
ref (refs/pull/NNN/merge) as the release target_commitish, which GitHub
rejects.

Add  condition so the draft-update step
only runs on push to main. The pull_request trigger is kept in the
block so the autolabeler still fires for PRs (it runs in a separate step
that is unaffected by this condition).

Author: Ritchie

.github/workflows/release-drafter.yml

@@ -25,6 +25,10 @@ jobs:
       # otherwise, read permission is required at least
       pull-requests: write
     runs-on: ubuntu-latest
+    # Only update the release draft on push to main, not on pull_request events.
+    # Running on PRs causes "Validation Failed: target_commitish" because
+    # release-drafter tries to set refs/pull/NNN/merge as the release target.
+    if: github.event_name == 'push'
     steps:
       # (Optional) GitHub Enterprise requires GHE_HOST variable set
       #- name: Set GHE_HOST