See https://hydra.nixos.org/build/80998335.
Upstream doesn't support QT 5.11 ATM which broke compilation:
```
src/dialogs/savedialog.cpp: In constructor ‘SaveDialog::SaveDialog(QWidget*, Qt::WindowFlags)’:
src/dialogs/savedialog.cpp:37:34: error: invalid use of incomplete type ‘class QButtonGroup’
group = new QButtonGroup(this);
```
The Arch community recommends to use an older QT version to fix
this (https://aur.archlinux.org/packages/chessx/).
Furthermore the `QT_PLUGIN_PATH` wasn't set properly which broke the
runtime since QT coudln't find the `xcb` plugin:
```
qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
This application failed to start because no Qt platform plugin could be initialized.
Reinstalling the application may fix this problem.
```
Finally, some minor style fixes were made for consistent indentation.
Addresses #45960
37032db583