ci: switch zig setup action to mlugg/setup-zig (successor)

The macOS Apple Silicon release job (and others) were showing
this warning on the GitHub Actions UI:

  ##[warning]Failed to restore: Cache service responded with 400
  ##[warning]Failed to save: <h2>Our services aren't available
    right now</h2>...

The warning was emitted by the 'Install Zig' step using
goto-bus-stop/setup-zig@v2. That action has been DEPRECATED by
its author and is now labeled 'Please use mlugg/setup-zig
instead' in its action.yml. The cache: true default (the action
caches the zig tarball via actions/cache) was hitting GitHub's
cache service during a brief outage.

Switch to the recommended successor:
  - goto-bus-stop/setup-zig@v2  ->  mlugg/setup-zig@v2
  - version: 0.16.0             (unchanged — same zig version)

The new action:
  - is actively maintained
  - installs zig without the actions/cache integration (no more
    'Failed to save/restore' warnings on cache hiccups)
  - uses the same tarball+PATH approach

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

Author: gilbertwong96

.github/workflows/ci.yml

@@ -238,9 +238,10 @@ jobs:
 
       - 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
+        # BEAM. Install it via mlugg/setup-zig (the recommended
+        # successor to goto-bus-stop/setup-zig, which is now
+        # deprecated and hits GitHub's cache service unreliability).
+        uses: mlugg/setup-zig@v2
         with:
           version: 0.16.0