GH Actions: run CodeQL only on cron-job, not on pull requests

Analysis takes more than 10 minutes, which is quite long to
run, so change it to only run on a cron job.

Also changing the cron to run on Thursdays instead of Fridays;
no need to worry about security issues during the Weekend.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.github/workflows/codeql-analysis.yml

@@ -6,13 +6,22 @@
 name: "CodeQL"
 
 on:
-  push:
+#  push:
-    branches: [master]
+#    branches: [master]
-  pull_request:
+#  pull_request:
-    # The branches below must be a subset of the branches above
+#    # The branches below must be a subset of the branches above
-    branches: [master]
+#    branches: [master]
   schedule:
-    - cron: '0 9 * * 5'
+    #        ┌───────────── minute (0 - 59)
+    #        │ ┌───────────── hour (0 - 23)
+    #        │ │ ┌───────────── day of the month (1 - 31)
+    #        │ │ │ ┌───────────── month (1 - 12)
+    #        │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday)
+    #        │ │ │ │ │
+    #        │ │ │ │ │
+    #        │ │ │ │ │
+    #        * * * * *
+    - cron: '0 9 * * 4'
 
 jobs:
   analyze: