use macos-latest with new xcode

Author: isuruf

.github/workflows/multibuild.yml

@@ -21,7 +21,7 @@ jobs:
         INTERFACE64: ['', '1']
         MB_ML_VER: ['', 2010, 2014]
         include:
-          - os: macos-11.0
+          - os: macos-latest
             PLAT: arm64
             INTERFACE64: ''
             platform: [x64]
@@ -72,6 +72,8 @@ jobs:
         pip install virtualenv
     - name: Build and Install Wheels
       run: |
+        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
         source travis-ci/build_steps.sh
         echo "------ BEFORE BUILD ---------"
         before_build