fix: add darwin/macOS platform support for nix flake (fixes #1)

- Enable CGO on darwin (required for libproc)
- Set MACOSX_DEPLOYMENT_TARGET=12.0 for Go 1.25 crypto/x509 compatibility
- Add nix build CI on macOS (macos-14 = Apple Silicon)

.github/workflows/ci.yaml

@@ -36,3 +36,21 @@ jobs:
         with:
           version: latest
 
+  nix-build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-14]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: DeterminateSystems/nix-installer-action@main
+
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+
+      - name: nix flake check
+        run: nix flake check
+
+      - name: nix build
+        run: nix build
+