QT_PLUGIN_PATH is broken, Qt6 applications can't find adwaita-qt

[Tungsten842]

Describe the bug

After #182580, I have set:

  qt = {
    enable = true;
    style = "adwaita";
    platformTheme = "gnome";
  };

But any qt6 application says: QApplication: invalid style override 'adwaita' passed, ignoring it.
This is definitly related to QT_PLUGIN_PATH, because if I export QT_PLUGIN_PATH like this:

QT_PLUGIN_PATH=/nix/store/v2w2xragrciibdgjkkdjmr0vmmgb34lz-adwaita-qt-1.4.1/lib/qt-6/plugins

Everthing works fine.

Steps To Reproduce

Open any qt6 app

Notify maintainers

@bobby285271 @hedning @jtojnar @dasj19 @maxeaubrey @linsui

Metadata

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

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
- system: `"x86_64-linux"`
 - host os: `Linux 6.1.3-xanmod1, NixOS, 23.05 (Stoat), 23.05.20230111.6c8644f`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.12.0`
 - channels(root): `""`
 - channels(quantmint): `""`
 - nixpkgs: `/run/current-system/nixpkgs`

[lilyinstarlight]

I think this may have worked by accident in Qt5. Because I can't find documentation to confirm that this behavior is intentional, but Qt5 searches for plugins in every PATH entry + ../lib/[qt-prefix]/plugins, but Qt6 only searches directly in entries in QT_PLUGIN_PATH (or QT_QPA_PLATFORM_PLUGIN_PATH) and does not make up extra search paths based on PATH

Therefore since /run/current-system/sw/bin is in PATH, Qt5 searches /run/current-system/sw/lib/[qt-5-prefix]/plugins, even though it isn't specified in QT_PLUGIN_PATH

So we should probably be setting QT_PLUGIN_PATH for all versions in nixos/modules/config/qt.nix regardless, unless that behavior in Qt5 is intentional (even then, explicitly adding it to QT_PLUGIN_PATH is clearer)

[lilyinstarlight]

I've opened #211003 to fix this