iso-installer image is slow to build

We want to test the iso-installer image using OpenQA on every build.

Currently the installer takes a long time to build, see:

• https://gitlab.gnome.org/GNOME/gnome-build-meta/-/jobs/1415062 - 139 minutes 10 seconds

Possible causes:

• rebuilding things every time that could actually be cached
• producing images which are bigger than they need to be