neuron: fix issue with incorrectly prefixed python path

[adevress]

Motivation for this change

Additional patch for #17914

Things done

• Tested using sandboxing
  (nix.useChroot on NixOS,
  or option build-use-chroot in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[FRidh]

Thanks. There's some other things that would be good to improve as well but that can be done later (see #17679 and #18143 for a possible solution).