fix(firestore): Attempt to fix CI by explicitly selecting Xcode 16.4

  The Firestore CI has been failing with an error that suggests Xcode is confused about which iPhone 16 simulator to use.

  This commit attempts to resolve the issue by explicitly selecting Xcode 16.4 before the build step. It's not guaranteed that this will solve the
  problem, but it's a likely candidate for the root cause.

Author: ncooke3

.github/workflows/firestore.yml

@@ -375,6 +375,9 @@ jobs:
 
     - uses: ruby/setup-ruby@354a1ad156761f5ee2b7b13fa8e09943a5e8d252 # v1
 
+    - name: Select Xcode
+      run: sudo xcode-select -s /Applications/Xcode_16.4.app/Contents/Developer
+
     - name: Setup build
       run:  scripts/install_prereqs.sh Firestore ${{ matrix.target }} xcodebuild