Set nightly wheels build to run on a daily schedule

Updated the workflow trigger from `unstable` branch pushes to a daily cron schedule at 02:00 UTC. Adjusted .gitattributes to ensure consistency by prioritizing our fork's version of workflow files during merges.

Author: urban233

.gitattributes

@@ -1,3 +1,5 @@
 *.pkl binary
 # Always keep our fork's version of pyproject.toml
 pyproject.toml merge=ours
+# Always keep our fork's GitHub Actions workflows
+.github/workflows/build.yml merge=ours

.github/workflows/nightly_wheels.yaml

@@ -15,9 +15,8 @@
 name: Build Nightly Wheels
 
 on:
-  push:
-    branches:
-      - unstable
+  schedule:
+    - cron: "0 2 * * *"  # Every day at 02:00 UTC
 
 env:
   VCPKG_ROOT: ${{ github.workspace }}/vendor/vcpkg