macosx: Hardcode the Qt version to 5.5 for now.

The latest Qt version available in Homebrew is 5.11.x currently, but
this version no longer supports older Mac OS X versions:

  https://doc.qt.io/qt-5/supported-platforms-and-configurations.html

In order for the DMG build to remain compatible with some more older
Mac OS X versions, we hardcode the scripts to use Qt 5.5 for now.

This requires the following on the build box:

 $ brew install qt@5.5
 $ brew unlink qt
 $ brew link --force qt@5.5

With this change the DMG should work on at least Mac OS X 10.8, 10.9,
and 10.10 according to Qt docs (only tested on 10.11.6 / El Capitan).

The DMG also currently uses Python 3.7, which should work fine for
Mac OS X 10.9 (Mavericks) and later, according to Python docs
(again, only tested on 10.11.6 / El Capitan).