Motivation for this change

Build failed after #22357.

The upstream maintenance situation for vdirsyncer is a bit dire at the moment (see pimutils/vdirsyncer#790 for details). Therefore we now use @spk's fix-build-style branch, which seems to be best currently maintained version of vdirsyncer. This branch also includes the two patches from pimutils/vdirsyncer#779, that we used to apply.

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 nix-review --run "nix-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.

Notify maintainers

cc @matthiasbeyer