chessx: fix build (#46359)

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