buildPythonPackage: document why we always propagate python

[teto]

python applications should not propagate python in PATH but we do it
nevertheless to avoid rebuilds see #170887
for an explanation.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[veprbl]

I don't think that few extra rebuilds is such a big issue here. What's probably is a real issue is that those rebuilds might require the applications to stop providing the python modules and vice versa. Otherwise we may get double references if both the pythonPackage and its tooling are pulled in by a derivation.

[FRidh]

I don't think that few extra rebuilds is such a big issue here. What's probably is a real issue is that those rebuilds might require the applications to stop providing the python modules and vice versa. Otherwise we may get double references if both the pythonPackage and its tooling are pulled in by a derivation.

Simply wrapping applications with another derivation would mean a tiny additional derivation instead of a larger rebuild.