[chore] run ci on every sunday

- zmk breaks every now and then and it's super annoying for end users
  if they fork this and end up with a non-working CI job, I hope this
  makes it so that it's reported earlier

.github/workflows/build.yml

@@ -1,6 +1,11 @@
 name: le_chiff_ble build
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0'
 
 jobs:
   build: