I forgot to mention some atomicity issues the
current mkdir-p/perms has. Excerpt from IRC:

(15:17:25) mdevos: I see ‘mkdir-p/perms’ doesn't create the directory
and set the permissions atomically; there's a tiny window where a
freshly-created directory has the permissions that would be expected
from the umask. Is this something to be concerned about (and to be
fixed in the patch)?
(15:40:46) civodul: mdevos: it's a good idea to be concerned about
this, yes :-)
(15:41:27) civodul: in general, given that changes in (guix build utils)
take time to trickle in, we should be extra cautious about interfaces and
implementation details

This patch doesn't address these potential issues.
Also, %dovecot-activation has an anomalous mkdir-p/perms.