ci: install zig 0.16.0 and xz in release job

The release jobs were failing at the Burrito pre_check step:

  > You MUST have `zig` and `xz` installed to use Burrito, we
    couldn't find all of them in your PATH!

GitHub-hosted runners don't ship with zig (which Burrito needs at
a pinned version, 0.16.0) or xz (used to extract zig's tarball and
musl).

Added three new steps to the release job, copied from Burrito's
own upstream CI workflow:

1. Install Zig via goto-bus-stop/setup-zig@v2 with version 0.16.0
   (pinned in deps/burrito/lib/burrito.ex as @zig_version_expected).
2. Install xz via apt-get on Linux runners, brew on macOS runners.
3. Verify zig/xz are present and on PATH.

Also extended the build artifact cache to include
~/.cache/burrito_file_cache — Burrito's download cache — so
re-runs don't re-download zig/musl/openssl on every run.

Tested: actionlint clean, YAML parses, mix ci green, 162 tests
pass.

Author: gilbertwong96

.github/workflows/ci.yml

@@ -229,13 +229,39 @@ jobs:
           otp-version: ${{ env.OTP_VERSION }}
           elixir-version: ${{ env.ELIXIR_VERSION }}
 
+      - name: Install Zig
+        # Burrito needs a specific zig version (0.16.0) to cross-compile
+        # BEAM. Install it via goto-bus-stop/setup-zig which is the
+        # same action Burrito's own CI uses.
+        uses: goto-bus-stop/setup-zig@v2
+        with:
+          version: 0.16.0
+
+      - name: Install xz
+        # Burrito needs xz to extract zig's tarball and musl. xz-utils
+        # ships with the Linux build image but not always; install
+        # defensively.
+        if: runner.os == 'Linux'
+        run: sudo apt-get -y install xz-utils
+
+      - name: Install xz (macOS)
+        # xz is rarely preinstalled on macOS runners.
+        if: runner.os == 'macOS'
+        run: brew install xz
+
+      - name: Verify zig/xz
+        run: |
+          zig version
+          xz --version
+
       - name: Cache build artifacts
         uses: actions/cache@v4
         with:
           path: |
             _build
             deps
             priv/plts
+            ~/.cache/burrito_file_cache
           key: release-${{ runner.os }}-${{ hashFiles('**/mix.lock') }}
           restore-keys: |
             release-${{ runner.os }}-