use new runner

.gitea/workflows/format.yml

@@ -1,4 +1,4 @@
-name: nix CI format
+name: nix format check
 on:
   push:
     branches: [main]
@@ -7,7 +7,7 @@ on:
 
 jobs:
   amd64:
-    runs-on: [ ubuntu-24.04, amd64-builder ]
+    runs-on: [ nix-flakes, amd64-builder ]
     steps:
       - name: System Info
         run: uname -m
@@ -21,9 +21,6 @@ jobs:
 
       - name: Checkout
         uses: actions/checkout@v5
-
-      - name: Install Nix using Determinate Systems action
-        uses: https://github.com/DeterminateSystems/nix-installer-action@v14
         
       - name: Alejandra Check
         run: nix run nixpkgs#alejandra -- --check .