Fix build issue when trying to run pack (#1942)

Build.cmd

 @echo off
-powershell -ExecutionPolicy ByPass -NoProfile -command "& """%~dp0eng\common\Build.ps1""" -restore -build %*"
+setlocal
+
+for %%x in (%*) do (
+    if "%%x"=="-pack" set _packArg=/p:BuildPackages=true
+)
+
+powershell -ExecutionPolicy ByPass -NoProfile -command "& """%~dp0eng\common\Build.ps1""" -restore -build %* %_packArg%"
 exit /b %ErrorLevel%

build.proj

-<Project Sdk="Microsoft.Build.Traversal">
+<Project>
+
+  <Import Project="Sdk.props" Sdk="Microsoft.Build.Traversal" />
   
   <PropertyGroup>
     <BuildRestore Condition="'$(BuildRestore)'==''">true</BuildRestore>
@@ -36,4 +38,9 @@
 
       <ProjectReference Condition="'$(BuildPackages)' == 'true'" Include="@(_ProjectsToPackage)" Targets="Pack" />
   </ItemGroup>
+
+  <Import Project="Sdk.targets" Sdk="Microsoft.Build.Traversal" />
+
+  <!-- Undefine Pack target as this repo handles Pack during build based on a different property. -->
+  <Target Name="Pack" />
 </Project>

build.sh

@@ -12,5 +12,13 @@ while [[ -h $source ]]; do
   [[ $source != /* ]] && source="$scriptroot/$source"
 done
 
+for arg in "$@"
+do
+  case "$arg" in
+  -pack)    _packArg=/p:BuildPackages=true
+            ;;
+  esac
+done
+
 scriptroot="$( cd -P "$( dirname "$source" )" && pwd )"
-"$scriptroot/eng/common/build.sh" --build --restore $@
+"$scriptroot/eng/common/build.sh" --build --restore $@ $_packArg