workflows/labels: disable scheduled runs in forks

This can still be manually dispatched for testing in forks, but it's
entirely useless to keep running it on schedule.

Also removing the "skip treewide" condition, which was a left-over and
removed everywhere else already. We don't want to skip any jobs,
especially not when considering required status checks.

(cherry picked from commit 075dc097a3)

.github/workflows/labels.yml

@@ -36,7 +36,7 @@ jobs:
   labels:
     name: label-pr
     runs-on: ubuntu-24.04-arm
-    if: "!contains(github.event.pull_request.title, '[skip treewide]')"
+    if: github.event_name != 'schedule' || github.repository_owner == 'NixOS'
     steps:
       - name: Install dependencies
         run: npm install @actions/artifact