Motivation for this change

Fixes #56042.
Krita no longer complains about not finding PyQt5 and sip (and in consequence no longer segfaults on exit).

Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'sip'
krita.scripting: "Traceback (most recent call last):"
krita.scripting: "  File \"/nix/store/0a81gj7kskr0nbpd90x93n08xk8snw75-krita-4.1.8/lib/krita-python-libs/krita/__init__.py\", line 7, in <module>"
krita.scripting: "    from .api import *"
krita.scripting: "  File \"/nix/store/0a81gj7kskr0nbpd90x93n08xk8snw75-krita-4.1.8/lib/krita-python-libs/krita/api.py\", line 28, in <module>"
krita.scripting: "    from PyKrita.krita import *"
krita.scripting: "ModuleNotFoundError: No module named 'PyQt5'"
krita.scripting: "Could not import krita"

Things done

Added a pythonpath wrapper for the krita and kritarunner binaries.
See the postInstall phase of pyqt5.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

Things to do/consider

• PyQt5 could be added to pythonpath instead of softlinking it... Are there any guidelines on how such things should be handled?
• Krita still complains about missing icons, we should probably add them it's actually not relevant
• By default krita expects the font Source Sans Pro to be installed, I'm not sure if it should be included in the package though
• Run some krita python scripts. (only quickly confirmed that Tools -> Scripts -> Scripter works)