ci: add nix build test

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: aster-void

.github/workflows/ci.yml

@@ -23,3 +23,10 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
       - run: cargo test
+
+  nix:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@main
+      - run: nix run . -- --help

flake.nix

@@ -21,6 +21,7 @@
           version = "0.1.0";
           src = ./.;
           cargoLock.lockFile = ./Cargo.lock;
+          doCheck = false;
           nativeBuildInputs = [ pkgs.makeWrapper ];
           postInstall = ''
             wrapProgram $out/bin/rollcron \