pureos-isohybrid.yaml: Integrate Librem 5 flash image type

This integrates the Librem 5 work from !2 with the work to build LUKS encrypted images from !1 (merged).

This now builds plain and LUKS images for both Librem 11 and Librem 5. A server (no GUI) amd64 flash image is also added.

All the images build, and most have built via Laniakea, but I have not tested all of them yet.

Still to do to merge:

• Review pureos/core/pureos-meta!7 (merged) (sudo, zstd in particular)
• Migrate pureos-minimal as well as debootstrap, flash-kernel, flash-root-init
• Remove hacks pulling in bleeding edge debs for those packages
• Build all images in Laniakea and test on hardware

After merge I will set up automatic triggers for Laniakea via crontab (weekly) and also have it keep only the latest image to avoid filling disks. We'll copy stable images to storage.puri.sm, Laniakea will only hold weekly images. Praveen has excluded Laniakea's directory for weekly images from backups, so we won't fill up backup disks either.

Closes #1, #2, #8