Use Xcode 12.0 for watchOS test app (#7101)

This ensures the iPhone 11 Pro simulator is availabl.e

.github/workflows/datatransport.yml

@@ -47,6 +47,8 @@ jobs:
     runs-on: macOS-latest
     steps:
     - uses: actions/checkout@v2
+    - name: Xcode 12
+      run: sudo xcode-select -s /Applications/Xcode_12.app/Contents/Developer
     - name: Setup Bundler
       run: scripts/setup_bundler.sh
     - name: Prereqs