Some more qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found. errors

[piegamesde]

Describe the bug

Some QT applications (I only tested Kile, but it's probably some more) are broken on latest master.

To Reproduce
Steps to reproduce the behavior:

NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/90321a7bb31f129ce6b0594fa5614c7a83c72528.tar.gz nixGLIntel nix-shell -p 'kile' --pure --run kile

Error message:

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Additional context

This is similar to #99937. Also some of the discussion in #99956

Notify maintainers

@FRidh

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

• system: "x86_64-linux"
• host os: Linux 5.8.10-arch1-1, Arch Linux, noversion
• multi-user?: no
• sandbox: yes
• version: nix-env (Nix) 2.3.7
• channels(user): "home-manager, nixpkgs-21.03pre244772.502845c3e31"
• nixpkgs: ~/.nix-defexpr/channels/nixpkgs

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

[FRidh]

fixed on master with c5a41da and on release-20.09 with 5f9c8a1.

I did notice the theme was not OK for me on master, so we may need to downgrade to Qt 5.12 there as well. Let me know if there are any more issues!

[piegamesde]

Thanks for the quick fix.