The package set is an attribute of the interpreter. The function to
build an environment (`buildEnv`/`withPackages`) is part of the
interpreter. The interpreter is passed to itself, and needs to be
updated when overridden.
For cross-compilation we splice the package set, and for that the
various `build/host` interpreters and sets need to be available. We
select these currently through `pkgs.${pythonAttr}`. The `pythonAttr`
attribute was not fixed for `pythonFull`.
https://github.com/NixOS/rfcs/pull/83https://github.com/NixOS/nixpkgs/pull/104201
We need a better solution for this because this is very brittle.
0dc64d5d71