Add nix CI (#28036)

This adds a nix CI job to build the flake in debug mode for
aarch64-darwin and x86-linux. For now this job will only run when the
`run-nix` label is added to a PR.

The CI job doesn't push to cachix for now, so every build is a clean
build.

I also added a condition to the garbage collection step so it only runs
when the nix store is >50GB.

Release Notes:

- N/A

.github/workflows/ci.yml

@@ -706,6 +706,51 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
+  nix-build:
+    timeout-minutes: 60
+    name: (${{ matrix.system.os }}) Nix Build
+    continue-on-error: true
+    if: github.repository_owner == 'zed-industries' && contains(github.event.pull_request.labels.*.name, 'run-nix')
+    strategy:
+      fail-fast: false
+      matrix:
+        system:
+          - os: x86 Linux
+            runner: buildjet-16vcpu-ubuntu-2204
+            install_nix: true
+          - os: arm Mac
+            runner: [macOS, ARM64, test]
+            install_nix: false
+    runs-on: ${{ matrix.system.runner }}
+    env:
+      ZED_CLIENT_CHECKSUM_SEED: ${{ secrets.ZED_CLIENT_CHECKSUM_SEED }}
+      ZED_CLOUD_PROVIDER_ADDITIONAL_MODELS_JSON: ${{ secrets.ZED_CLOUD_PROVIDER_ADDITIONAL_MODELS_JSON }}
+      GIT_LFS_SKIP_SMUDGE: 1 # breaks the livekit rust sdk examples which we don't actually depend on
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+        with:
+          clean: false
+      - name: Set path
+        if: ${{ ! matrix.system.install_nix }}
+        run: |
+          echo "/nix/var/nix/profiles/default/bin" >> $GITHUB_PATH
+          echo "/Users/administrator/.nix-profile/bin" >> $GITHUB_PATH
+
+      - uses: cachix/install-nix-action@02a151ada4993995686f9ed4f1be7cfbb229e56f # v31
+        if: ${{ matrix.system.install_nix }}
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
+        with:
+          name: zed-industries
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+          skipPush: true
+      - run: nix build .#debug
+      - name: Limit /nix/store to 50GB
+        run: '[ $(du -sm /nix/store | cut -f1) -gt 50000 ] && nix-collect-garbage -d'
+
   auto-release-preview:
     name: Auto release preview
     if: |