Use `curl` rather than `git` to download repositories

[RealOrangeOne]

Curl is much more efficient, and faster

[derneuere]

Thank you for your pull request!

While I do believe that there is a small performance win here, I prefer the readability of just using git.

[RealOrangeOne]

It's not just the performance - the container also ends up smaller as there's less extra metadata to persist. Although yes, the git command is much shorter!