fixes c88f3adb17, which resulted in
qt 5.15 being used in pythonPackages, despite 5.14 being
declared, and adapts qutebrowser accordingly.
'callPackage { pkgs = pkgs // { … }; }' does not work, because
it does not take into account the recursive evaluation of nixpkgs:
`pkgs/development/interpreters/python/default.nix` calls
`pkgs/top-level/python-packages.nix` with `callPackage`.
Thus, even if the former gets passed the updated `pkgs`,
the latter always gets passed `pkgs.pkgs`.
For the change in the qt5 version to apply consistently, 'pkgs.extend'
must be used.
qutebrowser only used the right qt5 version (5.15) because all
pythonPackages used it anyway.
7abb57c7b5