Split restore and build to fix design time imports

The restore and build target invocations need to happen in separate
msbuild task invocations and different global properties need to be
passed in to mitigate issues with caching the invocations. For more
context see dotnet/msbuild#2811.

eng/Build.props

@@ -75,8 +75,11 @@
     </ItemGroup>
 
     <MSBuild Projects="@(RepoTaskProjects)"
-             Properties="Configuration=Debug;Platform=AnyCPU"
-             Targets="Restore;Build"/>
+             Properties="Configuration=Debug;Platform=AnyCPU;__BuildPhase=Restore"
+             Targets="Restore" />
+    <MSBuild Projects="@(RepoTaskProjects)"
+             Properties="Configuration=Debug;Platform=AnyCPU;__BuildPhase=Build"
+             Targets="Build"/>
 
     <WriteLinesToFile File="$(RepoTasksOutputFile)"
                       Lines="$(RepoTasksOutputFile)"

src/coreclr/tests/src/runtest.proj

@@ -140,11 +140,18 @@ $(_XunitEpilog)
 
   <Target Name="BuildXunitWrapper">
     <!--
-      Restore and Build tasks shouldn't run in the same msbuild invocation as that could cause
-      issues with nuget auto-generated props/targets.
+      Restore and Build tasks shouldn't run in the same msbuild invocation as that could cause issues
+      with nuget auto-generated props/targets. https://github.com/microsoft/msbuild/issues/2811 for more context.
     -->
-    <MSBuild Projects="$(XunitWrapperSrcDir)\$(XunitWrapper).csproj" Targets="Restore" />
-    <MSBuild Projects="$(XunitWrapperSrcDir)\$(XunitWrapper).csproj" Targets="Build" />
+    <ItemGroup>
+      <XUnitWrapperProjectFile Include="$(XunitWrapperSrcDir)\$(XunitWrapper).csproj" />
+    </ItemGroup>
+    <MSBuild Projects="@(XUnitWrapperProjectFile)"
+             Targets="Restore"
+             Properties="__BuildPhase=Restore" />
+    <MSBuild Projects="@(XUnitWrapperProjectFile)"
+             Targets="Build"
+             Properties="__BuildPhase=Build" />
   </Target>
 
   <Import Project="$(MSBuildThisFileDirectory)../testgrouping.proj" />