Adding prop for CreateNuGetPackage target for SDK automation in RestAPI CI (Azure#20193)

Author: allenjzhang

build.proj

@@ -1,4 +1,6 @@
 <Project Sdk="Microsoft.Build.Traversal">
+  <Import Project="eng\mgmt\Directory.Build.Mgmt.props" />
+
   <ItemGroup>
     <ProjectReference Include="eng\mgmt.proj">
       <AdditionalProperties Condition="'$(Scope)' != ''">Scope=$(Scope)</AdditionalProperties>