move UWP PRs to hosted machines (#2562)

* try to move PRs to hosted machines

* hosted machine already has yarn

* fix path

.ado/windows-vs-pr.yml

@@ -29,8 +29,7 @@ jobs:
           BuildConfiguration: Release
           BuildPlatform: arm
     pool:
-      name: OE Standard Pool
-      demands: ['VisualStudio_15.0', 'OnPrem -equals False']
+      vmImage: vs2017-win2016
     timeoutInMinutes: 120 # how long to run the job before automatically cancelling
     cancelTimeoutInMinutes: 5 # how much time to give 'run always even if cancelled tasks' before killing them
     steps:
@@ -41,7 +40,19 @@ jobs:
         submodules: false # set to 'true' for a single level of submodules or 'recursive' to get submodules of submodules
         persistCredentials: false # set to 'true' to leave the OAuth token in the Git config after the initial fetch
 
-      - template: templates/npm-install-and-build.yml
+      - task: CmdLine@2
+        displayName: yarn install
+        inputs:
+          script: yarn install --frozen-lockfile
+          workingDirectory: vnext
+
+      - template: templates/stop-packagers.yml
+
+      - task: CmdLine@2
+        displayName: yarn build
+        inputs:
+          script: yarn build
+          workingDirectory: vnext
 
       - task: NuGetCommand@2
         displayName: NuGet restore - ReactWindows-UWP