absolute submodule URLs are now supported in CI since gitlab-runner
v15.11.0, see https://gitlab.com/gitlab-org/gitlab-runner/-/blob/main/CHANGELOG.md?ref_type=heads#new-features-18

also drop the note about the need to build lektor twice for lektor-scss,
this is inaccurate when the plugin is properly shipped, either via
packages/ or the virtualenv (but not both)