fedora-bot: Only auto-merge on Wednesdays

Currently we do excessive polling (every 30mins every day). Since our
releases are scheduled to always run on Wednesdays, let's rely on that
fact and define an ample enough window for PRs to get caught.

This means that when releasing on any other day (e.g. manually), one
needs to trigger the bot, merge the pagure PRs manually or wait until
the next Wednesday.

.github/workflows/poll-releases.yml

@@ -1,10 +1,11 @@
-# Poll for new releases in Fedora dist-git and schedule Koji builds and/or update Bodhi
+# Poll for new pull requests in Fedora dist-git auto-merge when green
 name: "Fedora bot"
 
 on:
   workflow_dispatch:
   schedule:
-    - cron: '*/30 * * * *'
+    # Every 30mins between 8AM and 3PM on Wednesdays
+    - cron: '*/30 8-15 * * 3'
 
 jobs:
   check: