CI: Use Xcode 13.0

Workaround for the configuration error on if_python

.github/workflows/ci-macvim.yaml

@@ -5,12 +5,12 @@ on:
   pull_request:
 
 env:
-  MACOSX_DEPLOYMENT_TARGET: 10.9
+  MACOSX_DEPLOYMENT_TARGET: "10.9"
 
   CC: clang
 
-  VERSIONER_PERL_VERSION: 5.18
-  VERSIONER_PYTHON_VERSION: 2.7
+  VERSIONER_PERL_VERSION: "5.18"
+  VERSIONER_PYTHON_VERSION: "2.7"
   vi_cv_path_python: /usr/bin/python
   vi_cv_path_python3: /usr/local/bin/python3
   vi_cv_path_plain_lua: /usr/local/bin/lua
@@ -39,9 +39,10 @@ jobs:
       matrix:
         include:
           - os: macos-10.15
-            xcode: 11.7
+            xcode: "11.7"
           - os: macos-10.15
           - os: macos-11
+            xcode: "13.0"
             publish: true
 
     runs-on: ${{ matrix.os }}