Explicitly select Xcode 15 in GitHub build script

Author: danielsaidi

.github/workflows/swift.yml

@@ -17,6 +17,9 @@ jobs:
       - uses: swift-actions/setup-swift@65540b95f51493d65f5e59e97dcef9629ddf11bf
         with:
           swift-version: "5.8"
+      - uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: latest-stable
       - name: Build
         run: swift build -v
       - name: Run tests