ci: use git:// protocol for external submodules
ci: use git:// protocol for external submodules

This is workaround for
https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/2148

Given the fact we only read it, and the fact that commit ID is stored
inside our repo, it should not bring any new security problems.

CZ-NIC/deckard,CZ-NIC/deckard