Run CI on a weekly schedule

Probably don't need daily runs given the amount of changes. Run on Friday
mornings so we can catch clippy changes from Rust releases that happen
on Thursday relatively soon after. Also include merge group support.

Author: djc

.github/workflows/build.yml

@@ -3,7 +3,12 @@ name: rustls
 permissions:
   contents: read
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+  merge_group:
+  schedule:
+    - cron: '23 6 * * 5'
 
 jobs:
   build: