Invoke the "Sign" target explicitly

Author: jstedfast

azure-pipelines/build.yml

@@ -120,4 +120,4 @@ extends:
           displayName: 'Sign Mappings NuGet Package'
           inputs:
             solution: 'azure-pipelines\nuget-package.signproj'
-            msbuildArguments: '/v:diagnostic /p:OutDir=$(Build.ArtifactStagingDirectory)\Packages /p:UnsignedPackagesPath=$(Build.ArtifactStagingDirectory)\UnsignedPackages'
+            msbuildArguments: '/t:Sign /v:diagnostic /p:OutDir=$(Build.ArtifactStagingDirectory)\Packages /p:UnsignedPackagesPath=$(Build.ArtifactStagingDirectory)\UnsignedPackages'