Motivation for this change

Gnunet didn't have a release in years but they are preparing a new one. Also they seem to have switched from svn to git for development, so I added a new git package. I tested nothing but the actual build and the start of a few binaries so far since gnunet is somewhat convoluted and I didn't manage to get it running with the old version. Going forward I'd like to add a pkg switch to the gnunet service and try to get that up to date for the next release.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-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/)
• Fits CONTRIBUTING.md.