Fix SourceRoot when NUGET_PACKAGES is set

Author: manfred-brands

src/Directory.Build.props

@@ -25,7 +25,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <SourceRoot Include="$(NuGetPackageRoot)" />
+    <SourceRoot Include="$([MSBuild]::EnsureTrailingSlash($(NuGetPackageRoot)))" />
   </ItemGroup>
 
   <Target Name="CoverletGetPathMap"
@@ -36,4 +36,4 @@
       <_LocalTopLevelSourceRoot Include="@(SourceRoot)" Condition="'%(SourceRoot.NestedRoot)' == ''"/>
     </ItemGroup>
   </Target>
-</Project>
+</Project>