Ensure rust is up to date in action

Author: Kuuuube

.github/workflows/nix.yml

@@ -8,6 +8,10 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
+    - name: Setup rust
+      run: |
+        rustup update stable
+        rustup default stable
     - uses: actions/checkout@v6
     - uses: cachix/install-nix-action@v31
       with: