disk-image and partitions

[rendaw <7e9wc56emjakcm@s.rendaw.me>]

So it looks like the root uuid might be generated, but from what I read
vfat uuids are translated from a partition serial number that isn't
manipulated in the guix source so getting uuids beforehand might be
impossible.

Both the root and efi partitions get a label, so I might be able to use
those to identify the partitions.  They're both hardcoded, but the
former is unstable (was changed earlier this month) and the definition
that exposes it (root-label) isn't exported...

I think my best choice here is to forcibly import "root-label" and
duplicate the hardcoded "GNU-ESP" in my config, but if the latter
changes I'll get an error at runtime rather than build time.

How would disk-image work in the first place?  It doesn't look like the
generated GNU-ESP or / are added to fstab automatically (checking the
output image).