ensure nix is available

Author: indirect

.github/workflows/test.yml

@@ -77,4 +77,7 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: actions-rust-lang/setup-rust-toolchain@v1
+      - uses: cachix/install-nix-action@v31
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
       - run: bin/kettle-build ${{ matrix.project }}