ci: Use semver tag for gitea-actions/nix-setup

Switch from @main to @v1 for stability. Major version tag auto-updates
to latest v1.x.x while avoiding breaking changes.

Closes: nix-7zf

.gitea/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
-      - uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@main
+      - uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@v1
 
       - name: Check flake
         run: nix flake check