Merge #246: ci: reduce nightly version update from semiweekly to weekly

b5433ca ci: reduce nightly version update from semiweekly to weekly (Andrew Poelstra)

Pull request description:

  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!
  
  Reduces the noise from the nightly-update PRs by half.


ACKs for top commit:
  delta1:
    utACK b5433ca


Tree-SHA512: b23d71f452d980fae5cc5e21c638ecea5875173579d690cf23cbb4dd034c2161e52226874e9e262d8dfd8e345f01a10440f1955689a3039af2050c41d4e50e4f

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: