[CI] Fix the CI error on macOS

CI/AzurePipelines/ContinuousBuild.yml

@@ -7,6 +7,12 @@ steps:
     git config --global user.name "Dummy Name"
   displayName: 'Config git'
 
+- task: UsePythonVersion@0
+  inputs:
+    versionSpec: '3.x'
+    addToPath: true
+    architecture: 'x64'
+
 - task: PythonScript@0
   displayName: 'Build'
   inputs:

azure-pipelines.yml

@@ -251,14 +251,13 @@ stages:
 
   - job: macOS_10_clang
     pool:
-      vmImage: macOS 10.14
+      vmImage: macOS-10.15
 
     variables:
       compiler: clang9
       combination: 'osx-$(compiler)-$(platform)-$(configuration)'
       buildFolder: 'Build/ninja-$(combination)'
       installCommand: |
-        brew update
         brew install ninja
       testCommand: './$(buildFolder)/Bin/ShaderConductorTest'
       artifactBinaries: |