nix: add GitHub Actions workflow

This adds a nix workflow that does the following:

* Checks to see if the Zig cache fixed derivation needs its hash
  updated. It does this by downloading a new cache, calculating the
  hash, and comparing it against what has already been set. If the hash
  is different, the workflow fails.
* Runs a Nix build if the hash is OK, testing the build of
  packages.ghostty (using "nix build .")

The cache workflow we use in the build job comes from:
  https://github.com/DeterminateSystems/magic-nix-cache-action/

Fixes #937.

.github/workflows/nix.yml

@@ -0,0 +1,30 @@
+on: [push, pull_request]
+name: Nix
+jobs:
+  check-zig-cache-hash:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Setup Nix
+        uses: cachix/install-nix-action@v23
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - name: Check Zig cache hash
+        run: nix develop -c ./nix/build-support/check-zig-cache-hash.sh
+  build:
+    needs: check-zig-cache-hash
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Setup Nix
+        uses: cachix/install-nix-action@v23
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - name: Setup Nix cache
+        uses: DeterminateSystems/magic-nix-cache-action@main
+        with:
+          diagnostic-endpoint: "" # disable telemetry
+      - name: Run build
+        run: nix build .