ci: run nix workflow on all PRs, not just this repos

Author: cachebag

.github/workflows/nix.yml

@@ -13,7 +13,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
         with:
-          ref: ${{ github.head_ref }}
           fetch-depth: 0
 
       - name: Install Nix