ci: add missing permission to add comments

While this worked during testing, the repository of a organization requires
extra permissions. Add write access to pull requests, like we do over at
packages.git.

Signed-off-by: Paul Spooren <mail@aparcar.org>

.github/workflows/build-on-comment.yml

@@ -4,6 +4,9 @@ on:
   issue_comment:
     types: [created, edited]
 
+permissions:
+  pull-requests: write
+
 concurrency:
   group: build-on-comment-${{ github.event.issue.number || github.event.pull_request.number }}
   cancel-in-progress: true