Update XCode version to one still present on runners.

List available here:
https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11-Readme.md#xcode

Author: DWesl

.github/workflows/multibuild.yml

@@ -76,8 +76,8 @@ jobs:
     - name: Build and Install Wheels
       run: |
         if [[ "$PLAT" == "arm64" ]]; then
-          sudo xcode-select -switch /Applications/Xcode_12.2.app
-          export SDKROOT=/Applications/Xcode_12.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk
+          sudo xcode-select -switch /Applications/Xcode_12.5.1.app
+          export SDKROOT=/Applications/Xcode_12.5.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk
         fi
         source travis-ci/build_steps.sh
         echo "------ BEFORE BUILD ---------"