flatpak-builder: Add io.qt.qtwebengine.BaseApp//5.15-21.08
Some applications are still using the 21.08 based runtime. Let's add it to the builder image until we've migrated everything to 5.15-22.08.
Some applications are still using the 21.08 based runtime. Let's add it to the builder image until we've migrated everything to 5.15-22.08.