ci: add workflow

.github/workflows/build.yml

@@ -0,0 +1,27 @@
+name: "build"
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v27
+      with:
+        github_access_token: ${{ secrets.GITHUB_TOKEN }}
+    - uses: DeterminateSystems/magic-nix-cache-action@v7
+    - uses: cachix/cachix-action@v15
+      with:
+        name: callumio-public
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+        extraPullNames: nix-community
+    - run: nix build
+    - run: nix flake check