Defer next flake.lock update till Dec.

Python 3.8 was recently removed from nixpkgs unstable in preparation for
the 24.05 release, whose support extends past py38's end-of-life date
coming up in October. Don't update to latest unstable before 24.11 comes
out to avoid losing py38 while we still support it.

.github/workflows/update_flake_lock.yml

@@ -4,7 +4,9 @@ name: update flake.lock
   workflow_dispatch:
   schedule:
     # Third day of every third month at noon
-    - cron: "0 12 3 */3 *"
+    #- cron: "0 12 3 */3 *"
+    # Wait till Dec 3 (by which point py38 will be EOL and Nix 24.11 will be out)
+    - cron: "0 12 3 12 *"
 
 jobs:
   main: