use flakes in ci

.github/workflows/nix-flake-check.yml

@@ -13,13 +13,14 @@ jobs:
     - uses: actions/checkout@v3
     - uses: cachix/install-nix-action@v20
       with:
-        nix_path: nixpkgs=channel:nixos-unstable
+        nix_path: nixpkgs=channel:nixos-stable
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+          system-features = nixos-test benchmark big-parallel kvm
     - uses: cachix/cachix-action@v12
       with:
         name: mycache
-        # If you chose signing key for write access
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
-        # If you chose API tokens for write access OR if you have a private cache
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-    - run: nix-build
+    - run: nix flake check
-    - run: nix-shell --run "echo OK"