bsdinstall: jail: Fix DISTMENU items

The menu was incorrectly using the fourth column (distname) instead of
the first (dist) of the MANIFEST.

The actual file name is on the first column of the MANIFEST file.
Remove the .txz part of the name to build the menu options.

Reviewed by: jamie
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D53177