Make sure to use Path.Combine when resolving the path

[mattleibow]

When trying to install packages into a portable class library on Mac OS X (Unix), the back slashes prevent NuGet from resolving the .NETPortable profiles.

Note: You do still need to set the NuGetPortableReferenceAssemblyPath environment variable.

[dnfclas]

@mattleibow, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;