I was having this issue with all of my submodules and altered the .git/config file so that it uses https:// as opposed to git://; however, this issue hasn’t resolved for mavlink.
I’m now on my second VM, I decided to try again from fresh in-case I messed up any of the VM setup.
I have attempted it without my Windows Firewall on.
I don’t understand why it would be using git:// after I altered the config file?
I’m not sure what other options I have to resolve this?
As well as your suggested git config --global url.https://github.com/.insteadOf git://github.com/ I also ran git config --global url.https://github.com/.insteadOf git@github.com/
I’m guessing there must be another git config file somewhere that is not ardupilot/.git/config