scripts: west: Run pristine.cmake directly instead of the target

When making a build folder pristine until now we were running the
'pristine' build target. The issue with that is that ninja/make or
whatever build tool is being used might decide to re-run CMake itself if
some of the dependencies have changes. This might trigger an error that
is unfriendly and unnecessary, since the user is explicitly asking for
the build folder to be wiped before starting a fresh build.
To avoid this issue restor to running directly the CMake script that the
'pristine' build target itself uses, so as to make sure that the build
folder is wiped unconditionally regardless of changes made to the tree.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>