#1905 breaks on Unix because of backslash

[sliepie]

In #1905 a backslash is added when the output param is set, as a fix for #896
Unfortunateley this breaks on Unix, as it expects a / rather than a \

@theimowski writes in 1905:

This seems to break older versions of mono, for
XBuild Engine Version 14.0
Mono, Version 4.6.1.0

xbuild tries to copy assemblies to a path with a backslash:

Copying file from '$BUILD_DIR/packages/NUnit/lib/nunit.framework.dll' to '$BUILD_DIR/test\nunit.framework.dll'

and fails to do so because that's a unix box.

We should detect if we're running on a Windows or a unix and and the correct slash to the end.