vm/windows: Add new runInWindowsVM function.

This function is quite similar to runInLinuxVM, but also ensures that
the builder is run decoupled of the Nix store and using the userland
inside the VM.

We're now picking up the environment variables saved in the previous
commit.

The reason we suppress all errors from the source operation is that it
would emit a ton of errors because we're trying to set read-only
variables.

Also, detecting whether the origBuilder is using the default builder
from the stdenv is currently a bit of a workaround until we have a
specialized pseudo-cross-stdenv someday in the future[TM].

Signed-off-by: aszlig <aszlig@redmoonstudios.org>