ci: remove submodule vendor folders to save space

Currently the `nimbus-build-system` exists twice in each checked out
`status-desktop` reposity, and each contains the `Nim-csources-v1`
submodule, which weights close to 900MB each:
https://github.com/nim-lang/csources_v1

Because of this each `status-destop` clone takes up 4 GB of disk space
after the cleanup has run, which is not okay.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

scripts/clean-git.sh

@@ -3,3 +3,4 @@
 git clean -qfdx
 git submodule foreach --recursive git reset -q --hard
 git submodule foreach --recursive git clean -qfdx
+git submodule foreach --recursive rm -fr vendor