loader: restore userboot help file

Commit e32fecd0c2c3 intended to skip installing all but one copy of
each loader variant's help file, but accidentally skipped all copies for
the userboot help file. (Other loaders install help files via the _simp
variant, but there is is no userboot_simp.)

PR: 271178
Fixes: e32fecd0c2c3 ("loader: install help files only once")
Sponsored by: The FreeBSD Foundation