Restore ability to deploy VSIX as a part of build

The VS SDK team is interested in tracking down the deployment errors
that we are seeing when deploying as a part of build.  This change adds
an option to make it easy to excercise that code path in builds.

Going to use this option to quickly toggle to the build based deployment
in order to mass test their fixes.

BuildAndTest.proj

@@ -15,51 +15,43 @@
     <RunTestArgs Condition="'$(TestVsiNetCore)' == 'true'">$(RunTestArgs) -trait:Feature=NetCore</RunTestArgs>
     <RunTestArgs Condition="'$(Trait)' != ''">$(RunTestArgs) -trait:$(Trait)</RunTestArgs>
     <RunTestArgs Condition="'$(NoTrait)' != ''">$(RunTestArgs) -notrait:$(NoTrait)</RunTestArgs>
+    <DeployExtensionViaBuild Condition="'$(DeployExtensionViaBuild)' == ''">false</DeployExtensionViaBuild>
     <IncludePattern Condition="'$(IncludePattern)' == '' AND '$(TestVsi)' != 'true'">*.UnitTests.dll</IncludePattern>
     <IncludePattern Condition="'$(IncludePattern)' == '' AND '$(TestVsi)' == 'true'">*.IntegrationTests.dll</IncludePattern>
     <OutputDirectory>$(MSBuildThisFileDirectory)Binaries\$(Configuration)\</OutputDirectory>
     <RunTestArgs>$(RunTestArgs) -log:&quot;$(OutputDirectory)\runtests.log&quot;</RunTestArgs>
     <CoreClrTestDirectory>$(OutputDirectory)\CoreClrTest</CoreClrTestDirectory>
-    <MSBuildCommonProperties>
-      RestorePackages=false;
-      TreatWarningsAsErrors=true;
-      DeployExtension=false;
-    </MSBuildCommonProperties>
   </PropertyGroup>
 
   <Target Name="Build">
     <MSBuild BuildInParallel="true"
              Projects="$(RoslynSolution)"
-             Properties="$(MSBuildCommonProperties)"
+             Properties="DeployExtension=$(DeployExtensionViaBuild)"
              Targets="Build" />
     <MSBuild BuildInParallel="true"
              Condition="'$(ManualTest)' == ''"
              Projects="$(SamplesSolution)"
-             Properties="$(MSBuildCommonProperties)"
+             Properties="DeployExtension=false"
              Targets="Build" />
   </Target>
 
   <Target Name="Clean">
     <MSBuild BuildInParallel="true"
              Projects="$(RoslynSolution)"
-             Properties="$(MSBuildCommonProperties)"
              Targets="Clean" />
     <MSBuild BuildInParallel="true"
              Condition="'$(ManualTest)' == ''"
              Projects="$(SamplesSolution)"
-             Properties="$(MSBuildCommonProperties)"
              Targets="Clean" />
   </Target>
 
   <Target Name="Rebuild">
     <MSBuild BuildInParallel="true"
              Projects="$(RoslynSolution)"
-             Properties="$(MSBuildCommonProperties)"
              Targets="Rebuild" />
     <MSBuild BuildInParallel="true"
              Condition="'$(ManualTest)' == ''"
              Projects="$(SamplesSolution)"
-             Properties="$(MSBuildCommonProperties)"
              Targets="Rebuild" />
   </Target>
 
@@ -117,7 +109,7 @@
   </Target>
 
   <!-- Manually deploy any VSIX required by our integration tests (https://github.com/dotnet/roslyn/issues/17456) -->
-  <Target Name="DeployRoslynVsix" Condition="'$(TestVsi)' == 'true'">
+  <Target Name="DeployRoslynVsix" Condition="'$(TestVsi)' == 'true' AND '$(DeployExtensionViaBuild)' != 'true'">
 
     <PropertyGroup>
       <VsixExpInstallerExe>$(NuGetPackageRoot)\roslyntools.microsoft.vsixexpinstaller\$(RoslynToolsMicrosoftVSIXExpInstallerVersion)\tools\VsixExpInstaller.exe</VsixExpInstallerExe>

build/Targets/Settings.props

@@ -45,6 +45,7 @@
     <VisualStudioReferenceAssemblyVersion Condition="'$(VisualStudioReferenceAssemblyVersion)' == ''">$(VisualStudioReferenceMajorVersion).0.0.0</VisualStudioReferenceAssemblyVersion>
     <VisualStudioCodename>Dev$(VisualStudioReferenceMajorVersion)</VisualStudioCodename>
     <MinimumVisualStudioVersion>$(VisualStudioVersion)</MinimumVisualStudioVersion>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
 
     <!-- Disable AppX packaging for the Roslyn source. Not setting this to false has the side effect
          that any builds of portable projects end up in a sub folder of $(OutputPath). Search for this flag in