It needs to be separate script, because once the submodules are broken
and the `variables.mk` file is not available from `nimbus-build-system`
then we cannot even call `make clean-git` successfully because it will
be caught by the `if` clause that checks for `variables.mk`.
Possible fix for issues with submodules not being updated in some
windows release builds.
Signed-off-by: Jakub Sokołowski <jakub@status.im>
