ci: fix Zig setup (mlugg/setup-zig@v2) and run Ubuntu only

v1 of the action does not support zig 0.16.0's tarball name format.
Single Ubuntu runner is enough; mvn verify still cross-compiles all six
native targets via zig.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

.github/workflows/ci.yml

@@ -8,14 +8,7 @@ on:
 
 jobs:
   build:
-    name: ${{ matrix.os }}
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        # each runner builds its own native lib via zig and tests against it:
-        #   ubuntu-latest -> linux-x86_64, macos-14 -> osx-aarch64, macos-13 -> osx-x86_64
-        os: [ubuntu-latest, macos-14, macos-13]
+    runs-on: ubuntu-latest
 
     steps:
       - name: Checkout (with zstd submodule)
@@ -31,7 +24,7 @@ jobs:
           cache: maven
 
       - name: Set up Zig
-        uses: mlugg/setup-zig@v1
+        uses: mlugg/setup-zig@v2
         with:
           version: 0.16.0