As of recently, Nix 2.6 from Ubuntu 22.04 became too old to evaluate
nixpkgs. A new-enough version of Nix is available as part of Ubuntu
24.04, but those newer versions of Nix aren't happy with our rather
primitive proot workaround anymore.
Thankfully, someone already made a version of Nix that does all the
heavy lifting for running in unprivileged environments like the one
Read the Docs provides. So we used that instead.