do not install dotnet.

use azure powershell tasks

azure-pipelines.yml

@@ -3,14 +3,7 @@ jobs:
   pool:
     vmImage: 'ubuntu-16.04'
   steps:
-  - task: DotNetCoreInstaller@0
-    inputs:
-      version: '2.1.403'
   - script: |
-      export COREHOST_TRACE=0
-      export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
-      export DOTNET_CLI_TELEMETRY_OPTOUT=1
-      which dotnet
       dotnet --info
       ./build.sh --target "Azure-Linux" --configuration "Release"
      
@@ -18,14 +11,7 @@ jobs:
   pool:
     vmImage: 'xcode9-macos10.13'
   steps:
-  - task: DotNetCoreInstaller@0
-    inputs:
-      version: '2.1.403'
   - script: |
-      export COREHOST_TRACE=0
-      export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
-      export DOTNET_CLI_TELEMETRY_OPTOUT=1
-      which dotnet
       dotnet --info
       ./build.sh --target "Azure-OSX" --configuration "Release"
 
@@ -33,16 +19,9 @@ jobs:
   pool:
     vmImage: 'vs2017-win2016'
   steps:
-  - task: DotNetCoreInstaller@0
+  - task: PowerShell@2
     inputs:
-      version: '2.1.403'
-  - script: |
-      set COREHOST_TRACE=0
-      set DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
-      set DOTNET_CLI_TELEMETRY_OPTOUT=1
-      where dotnet
-      dotnet --info
-      build.ps1 --target "Azure-Linux" --configuration "Release"
+      script: build.ps1 --target "Azure-Linux" --configuration "Release"
   - task: PublishBuildArtifacts@1
     inputs:
       pathToPublish: artifacts/