build: move nightly build schedule to separate workflow (#10000)

This allows users to easily disable nightly builds in their forks,
simply by disabling the
build-nightly action.

### Testing

I tested it in my fork, and it works.

.github/workflows/build-syncthing.yaml

@@ -3,9 +3,7 @@ name: Build Syncthing
 on:
   pull_request:
   push:
-  schedule:
-    # Run nightly build at 05:00 UTC
-    - cron: '00 05 * * *'
+  workflow_call:
   workflow_dispatch:
 
 env: