Add slash command access to make unit (#3186)

This should allow PR comments of the form:
`/retest unit` to trigger `make unit`

Signed-off-by: btofel <btofel@redhat.com>

.github/workflows/unit.yml

@@ -8,8 +8,12 @@ on:
   pull_request:
   workflow_dispatch:
   merge_group:
+  issue_comment:
+    types: [created]  # Triggers the workflow when a comment is created. see `if` section
 jobs:
   unit:
+    if: >-
+      github.event_name != 'issue_comment' || startsWith(github.event.comment.body, '/retest unit')
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3