I have no idea, to be honest. Information on how exactly this should be handled is sparse (the release is so new), espacially on git submodule since git submodules suck to begin with and barely anyone uses it, let along with file:. I can’t make the config work on my own machine either, so this is not specific to CI, but it’s unclear whether I’m missing something, or the git docs lacks critical information.

OK, so we're safe to say to people hit by this "we don't know how to get file: urls to work either, you'll need to ask in a git support channel". That sounds fair to me.