Make Nopipeline work with .Net Core on Linux

[jamsilva]

I am unable to use Nopipeline as-is on Linux. However, changing all backslashes to slashes in Nopipeline.Task.targets makes things work correctly.
I expect macOS to have the same problem and I think Windows doesn't care but I only tested this change on Linux.

[Martenfur]

Macos actually works as is. If you'll confirm that it works on both linux and Windows, feel free to make a pr. I'll look into it later.

[Martenfur]

Merged your PR. ; -)