Refactor run-tests.ps1 to not install .NET Core

[MichaelSimons]

It should not be necessary to install .NET Core within run-tests.ps1. This logic is not robust and will fail the builds periodically. Investigate other options such as building/running the tests within a container to eliminate this flakiness.

[mthalman]

This is specific to Windows which requires that tests be run directly on the build agent instead of a container.

[mthalman]

This wouldn't be necessary once dotnet/docker-tools#159 is fixed. This hasn't been causing any functional issues so I think it makes sense to not do any work here and just wait for the Windows nested container support.