Make sure we set VERSION so linuxdeployqt/appimagetool knows how to name the appimage.

Previously it did not do this, and we had our own logic to rename the
appimages (which is now unnecessary, and is removed in this fix)

Ref T10428

(cherry picked from commit e3c27df58f0a958b949b0aa0a9a6bfe905e80836)