workflows/check-nix-format: Allow testing in forks

It seems like imposed limitation came from ofborg not running in forks,
which doesn't apply for standard GitHub actions:
88c60d97fc

.github/workflows/check-nix-format.yml

@@ -15,7 +15,7 @@ permissions:
 jobs:
   nixos:
     runs-on: ubuntu-latest
-    if: "github.repository_owner == 'NixOS' && !contains(github.event.pull_request.title, '[skip treewide]')"
+    if: "!contains(github.event.pull_request.title, '[skip treewide]')"
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
         with: