update build fails on mtree invocation

[Rhialto <rhialto%falu.nl@localhost>]

This doesn't happen with a fresh build, but a re-build in update mode
fails like this (with -j 4, in case it matters):

obj ===> regress/lib/libdes
obj ===> regress/lib/libc/siginfo/sigfpe
obj ===> regress/lib/libc/siginfo/sigsegv
clean_METALOG ===> .
clean_METALOG ===> distrib/sets
nbmtree: unknown group `ftp'
nbmtree: failed at line 1301 of the specification
--- /vol-fourquid-1/olafs/netbsd-git//work/destdir.amd64/METALOG.sanitised ---
*** [/vol-fourquid-1/olafs/netbsd-git//work/destdir.amd64/METALOG.sanitised] 
Error code 1
1 error
nbmake: stopped in /tank/vol-fourquid-1/olafs/netbsd-git/cvs/src/distrib/sets
--- clean_METALOG ---
*** [clean_METALOG] Error code 2

-Olaf.
-- 
___ Olaf 'Rhialto' Seibert    -- You author it, and I'll reader it.
\X/ rhialto/at/xs4all.nl      -- Cetero censeo "authored" delendum esse.