ci: use master Zig build to fix 404 on 0.15.2

Author: cpbeamer

.github/workflows/zig-ci.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Setup Zig
         uses: mlugg/setup-zig@v1
         with:
-          version: 0.15.2
+          version: master
 
       - name: Check Formatting
         run: zig fmt --check .