use built-in nix on ci runner

.gitea/workflows/build.yaml

@@ -10,8 +10,6 @@ jobs:
       - name: Hello
         run: |
           echo "Hello and welcome"
-      - name: Install Nix
-        uses: https://github.com/DeterminateSystems/nix-installer-action@main
       - uses: actions/checkout@v3
         name: Checkout
       - name: Metadata

nixos/boxes/bolty/gitea-runner.nix

@@ -33,6 +33,7 @@ in
         nodejs
         wget
         sudo
+        nix
       ];
       labels = [
         "nixery-nix-node:docker://nixery.dev/shell/nix/git/nodejs:latest"