nightly: skip Fri/Sat night (#6242)

It's unlikely that anyone is looking at the results when there's a
fresh run's results (of Sunday) awaiting you on Monday.

Signed-off-by: Stephan Renatus <stephan@styra.com>

.github/workflows/nightly.yaml

@@ -2,7 +2,7 @@ name: Nightly
 on:
   workflow_dispatch: {}   # Allow for manual triggers
   schedule:
-    - cron:  '0 8 * * *'  # Daily, at 8:00 UTC
+    - cron:  '0 8 * * 0-4' # Sun-Thu, at 8:00 UTC
 
 
 jobs: