Adjust for renaming of pythonForBuild to pythonOnBuildForHost

[sigprof]

The upstream Nixpkgs renamed pythonForBuild to pythonOnBuildForHost and made pythonForBuild emit warnings: NixOS/nixpkgs#265764. Adjust the poetry2nix code to avoid warnings with the new version, while still keeping compatibility with the old one.

Contribution checklist (recommended but not always applicable/required):

• There's an automated test for this change
• Commit messages or code include references to related issues or PRs (including third parties)
• Commit messages are conventional - examples from the log include "feat: add changelog files to fixup hook", "fix(contourpy): allow wheel usage", and "test: add sqlalchemy2 test"

[cpcloud]

Thanks!