nix: Add nightly build job with cachix (#27014)

I'll be using this to `nix run github:zed-industries/zed/nightly` and
get an up-to-date and cached nightly build.

It'll also serve as a way to warn me when the nix build is broken,
rather than having to wait for users to report it.

Eventually and depending on the build time of the nix builds, we may
want to consider putting a nix build in CI (#17458) to prevent
breakages, but for now a best-effort nightly build that doesn't block
the job if it fails is a good start.

Resolve #19937

Release Notes:

- N/A

.github/workflows/release_nightly.yml

@@ -170,6 +170,58 @@ jobs:
       - name: Upload Zed Nightly
         run: script/upload-nightly linux-targz
 
+  bundle-nix:
+    timeout-minutes: 60
+    name: (${{ matrix.system.os }}) Nix Build
+    continue-on-error: true
+    strategy:
+      fail-fast: false
+      matrix:
+        system:
+          - os: x86 Linux
+            runner: buildjet-16vcpu-ubuntu-2204
+            install_nix: true
+          - os: arm Mac
+            # TODO: once other macs are provisioned for nix, remove that constraint from the runner
+            runner: [macOS, ARM64, nix]
+            install_nix: false
+          - os: arm Linux
+            runner: buildjet-16vcpu-ubuntu-2204-arm
+            install_nix: true
+    if: github.repository_owner == 'zed-industries'
+    runs-on: ${{ matrix.system.runner }}
+    needs: tests
+    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
+
+      # on our macs we manually install nix. for some reason the cachix action is running
+      # under a non-login /bin/bash shell which doesn't source the proper script to add the
+      # nix profile to PATH, so we manually add them here
+      - 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 }}"
+      - run: nix build
+      - run: nix-collect-garbage -d
+
   update-nightly-tag:
     name: Update nightly tag
     if: github.repository_owner == 'zed-industries'