CI: Check URLs on schedule only to reduce false positives

Links occasionally fail due to network issues or other transient problems.
This should not cause pull requests to fail checks.

The check is now performed weekly at a random-looking time on Fridays
(when server load isn't too high).

.github/workflows/check_urls.yml

@@ -1,12 +1,10 @@
 name: 🌐 Check URLs
 on:
-  push:
-  pull_request:
   schedule:
-    # Every day at 18:00 UTC.
+    # Every Friday at 16:27 UTC.
     # URLs can decay over time. Setting up a schedule makes it possible to be warned
     # about dead links as soon as possible.
-    - cron: "0 18 * * *"
+    - cron: "27 16 * * FRI"
 
 jobs:
   check-urls: