add nixos check

add cachix action

.github/workflows/nixos.yml

@@ -0,0 +1,23 @@
+name: "NixOS-Tests"
+on:
+  pull_request:
+  push:
+jobs:
+  nixos-flake-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2.3.4
+    - uses: cachix/install-nix-action@v14
+      with:
+        install_url: https://nixos-nix-install-tests.cachix.org/serve/vij683ly7sl95nnhb67bdjjfabclr85m/install
+        install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+    - uses: cachix/cachix-action@v10
+      with:
+        name: bcachefs-tools
+        # If you chose API tokens for write access OR if you have a private cache
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: nix flake show
+    - run: nix flake check --print-build-logs