Motivation for this change

The addon manager failed to update the addon list, because GitPython was missing. Maybe we should switch to a separate wrapper package at some point, but I haven't got the time for that atm.

The first commit should be ported back to stable.

I've also removed the freecadStable package, which no longer builds since qtwebkit has been marked insecure.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.