Test build and run Nix Flake in CI (#1477)

Co-authored-by: figsoda <figsoda@pm.me>

.github/workflows/ci.yml

@@ -1,6 +1,10 @@
 name: Continuous integration
 on: [push, pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   RUSTFLAGS: "-Dwarnings"
   RUSTDOCFLAGS: "-Dwarnings"
@@ -35,3 +39,15 @@ jobs:
       - uses: dtolnay/rust-toolchain@1.70.0
       - uses: Swatinem/rust-cache@v2
       - run: cargo check --workspace
+
+  nix-flake:
+    name: Test Nix Flake
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: DeterminateSystems/nix-installer-action@v4
+      - uses: DeterminateSystems/magic-nix-cache-action@v2
+      - run: nix flake check
+      - run: nix build
+      - run: nix shell -c typst --version
+      - run: nix run . -- --version