ci: add cron schedule trigger

Also moves/reformats `on` triggers to match other repos.

Author: cpu

.github/workflows/build.yml

@@ -1,6 +1,12 @@
-on: [push, pull_request, merge_group]
 name: rustls-pemfile
 
+on:
+  push:
+  pull_request:
+  merge_group:
+  schedule:
+    - cron: '0 18 * * *'
+
 jobs:
   rustfmt:
     runs-on: ubuntu-latest