documentation: change git:// links to https://

[mat8913]

Motivation for this change

The server is not verified over the git:// transfer protocol. If you
clone a repository over git://, you should check if the latest commit's
hash is correct.

On the other hand, https:// will always verify the server automatically,
using certificate authorities.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.