ci: check nix flake

Author: eopb

.github/workflows/nix.yml

@@ -0,0 +1,37 @@
+name: Nix
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["*"]
+
+permissions: read-all
+
+jobs:
+  check:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-14]
+    runs-on: ${{ matrix.os }}
+
+    name: flake check
+    steps:
+      - uses: actions/checkout@v4.1.7
+        with:
+          fetch-depth: 0
+      - uses: DeterminateSystems/nix-installer-action@v13
+      - uses: DeterminateSystems/magic-nix-cache-action@v7
+      - run: nix flake check -L --show-trace
+  fmt:
+    runs-on: ubuntu-latest
+
+    name: nix fmt
+    steps:
+      - uses: actions/checkout@v4.1.7
+        with:
+          fetch-depth: 0
+      - uses: DeterminateSystems/nix-installer-action@v13
+      - uses: DeterminateSystems/magic-nix-cache-action@v7
+      - run: nix run nixpkgs#nixfmt-rfc-style -- --check flake.nix