Make nupkg signing exclusion conditional

Author: dibarbet

eng/Signing.props

@@ -19,8 +19,6 @@
     <ItemsToSign Include="$(ArtifactsPackagesDir)**\*.tgz" />
     <ItemsToSign Include="$(VisualStudioSetupOutputPath)**\*.zip" />
 
-    <FileExtensionSignInfo Update=".nupkg" CertificateName="None" />
-
     <!-- Expression Compiler dependencies that run in remote debugger must be signed with Microsoft101240624 certificate in order to work on Windows 10S -->
 
     <FileSignInfo Include="Microsoft.CodeAnalysis.ExpressionEvaluator.ExpressionCompiler.dll" CertificateName="Microsoft101240624"/>
@@ -64,4 +62,8 @@
     <FileSignInfo Include="StreamJsonRpc.dll" CertificateName="MicrosoftSHA2" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(PreReleaseVersionLabel)' == 'pr-validation' And '$(SignType)' == 'test'">
+    <FileExtensionSignInfo Update=".nupkg" CertificateName="None" />
+  </ItemGroup>
+
 </Project>