Motivation for this change

This fixes #87642, allowing U2F to work during OAUTH2 authentication flows, which is necessary for Thunderbird to work with Google accounts which are set to require U2F as an authentication method. The fix works by ensuring that the systemd lib directory is in Thunderbird's rpath, thus allowing Thunderbird to locate libudev at runtime.

I first tried to do this by symlinking libudev, as several other packages do, but it didn't work and I don't know why. This approach does work, so it has that going for it, which is nice.

Given how subtle this issue is, it would be really nice to have an automated test for it, to prevent regressions. To write one, though, I would need to spin up a web server running some form of U2F code, and script the Thunderbird UI heavily. I'm not aware that there's any testing framework in nixpkgs for that sort of thing. So I'm not going to do that work, but if somebody with more experience writing testing harnesses in nixpkgs wants to take it on, I'd be happy to advise.

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.