Allow Qt 5 and Qt 6 to coexist

[pluiedev]

Describe the bug

Currently Qt 5 and Qt 6 packages cannot be placed together in the build inputs of a derivation, as an error would occur very early on in the build:

nimf> building '/nix/store/i3h1hjvpbng9lfccrvw5xpgk8mli95a6-nimf-1.3.8.drv'
nimf> Error: detected mismatched Qt dependencies:
nimf>     /nix/store/w9g781rqrc12870vji13hycwh47lwsgd-qtbase-6.8.0
nimf>     /nix/store/yzwd3xa66zrq4iihfj5vpxr3xzlsrphk-qtbase-5.15.15-dev

This limitation severely harms packages that expect both to be built at the same time, like for example Klassy and Nimf. While the former case can be partially mitigated by either manually specifying the paths to each and every Qt module for each version individually or by only building either the Qt 5 or Qt 6 build, for the latter there is simply no way to do it aside from massively rewriting its configure script.

Notify maintainers

@NixOS/qt-kde

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