Fix detection of the proxy base style when QT_STYLE_OVERRIDE is set

QProxyStyle checks QApplicationPrivate::styleOverride but ignores
QT_STYLE_OVERRIDE when it is trying to detect the name of the base
style.
Set styleOverride from QT_STYLE_OVERRIDE to make it work. Thus there
is no need to check QT_STYLE_OVERRIDE from multiple places.

Change-Id: I2a64b5ff5443701f800ef5d2a9cb425068f909f1
Task-number: QTBUG-52596
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>