ci: reduce frequency

Reduce the frequency from 14 times a week to 3 (08:00 UTC on Monday,
Wednesday and Friday), since the `rust` branch does not change that
often now, PRs run the CI too and other services are/will also be
testing the different branches (e.g. 0-DAY CI, KernelCI...).

Suggested-by: Alex Gaynor <[email protected]>
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

.github/workflows/ci.yaml

@@ -1,7 +1,7 @@
 on:
   pull_request: {}
   schedule:
-    - cron: '30 5,17 * * *'
+    - cron: '0 8 * * MON,WED,FRI'
   push:
 
 jobs: