add github action for nix build

Author: wd15

.github/workflows/nix.yml

@@ -0,0 +1,23 @@
+---
+name: Nix
+on: [push, pull_request]
+jobs:
+  test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        working-directory: ./
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+          extra_nix_config: |
+            trusted-public-keys = mdfbaam.cachix.org-1:WCQinXaMJP7Ny4sMlKdisNUyhcO2MHnPoobUef5aTmQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+            substituters = https://mdfbaam.cachix.org https://cache.nixos.org
+      - run: nix flake check
+      - run: nix build
+