ci(retest): filter more comments to use GHA less (argoproj#13208)

Signed-off-by: Anton Gilgur <agilgur5@gmail.com>

.github/workflows/retest.yaml

@@ -1,14 +1,15 @@
 name: Detect and Trigger Retest
 on:
   issue_comment:
-    types: [created, edited]
+    types: [created]
 
 permissions:
   contents: read
 
 jobs:
   retest:
-    if: github.event.comment.author_association == 'MEMBER' && github.event.comment.body == '/retest'
+    # PR comments where a Member types "/retest" exactly
+    if: github.event.issue.pull_request && github.event.comment.author_association == 'MEMBER' && github.event.comment.body == '/retest'
     permissions:
       actions: write # for re-running failed jobs: https://docs.github.com/en/rest/actions/workflow-runs?apiVersion=2022-11-28#re-run-a-job-from-a-workflow-run
     runs-on: ubuntu-latest