Fix GitHub Action: Use macOS 13 with Xcode 15.2 (Swift 5.9)

Swift 5.5 toolchain is incompatible with macOS 15.5 SDK on macos-latest.
Using macos-13 with Xcode 15.2 provides Swift 5.9.2 which is compatible
with our Swift 5.5+ codebase and the available SDK.

Author: rodchristiansen

.github/workflows/build-release.yml

@@ -14,16 +14,17 @@ on:
 jobs:
   build:
     name: Build munkipkg
-    runs-on: macos-latest
+    runs-on: macos-13
 
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
 
-      - name: Setup Swift
-        uses: swift-actions/setup-swift@v2
-        with:
-          swift-version: '5.5'
+      - name: Select Xcode 15.2 (Swift 5.9.2)
+        run: sudo xcode-select -s /Applications/Xcode_15.2.app/Contents/Developer
+      
+      - name: Verify Swift version
+        run: swift --version
 
       - name: Build release binary
         run: |