Add environment variable for 4.3.0 installer/maintenance tool RTA testing

The 4.3.0 installer release will contain a Qt baseline update from Qt 5.15.11 --> Qt 5.15.x.

RTA testing of the Qt baseline update requires that it's possible to select which version of the maintenance tool will be used in RTA. This can be handled via a specific environment variable.

Implementation of this can be started after https://bugreports.qt.io/browse/QTIFW-2388 has been done. Also, repositories need to be available (Katja, Arttu from the installer team will take care of this).