We have a bunch of NuGet.Config files in our tests that were needed before because the tests were run out of a temp directory. Now the tests are run out of the artifacts folder, which is a subdirectory of our repo root, so the root NuGet.Config will get picked up.
I also updated all the dotnet-core feeds from www.myget.org to dotnet.myget.org.
Rather than keep a map that will have to be constantly updated every time
a new argument gets added to a compiler, the 'additionalArguments' option
will allow users to directly add arguments to the underlying compiler.
