ci: reduce nightly version update from semiweekly to weekly

Turns out it's hard with GA to do things every other week (or every four weeks).
But it's easy to at least stop doing it twice every week!

Author: apoelstra

.github/workflows/cron-semi-weekly-update-nightly.yml renamed to .github/workflows/cron-weekly-update-nightly.yml

@@ -1,7 +1,7 @@
 name: Update Nightly rustc
 on:
   schedule:
-    - cron: "5 0 * * 1,4" # runs every Monday and Thursday at 00:05 UTC
+    - cron: "5 0 * * 1" # runs every Monday at 00:05 UTC
   workflow_dispatch: # allows manual triggering
 jobs:
   format: