make vi message catalogues build independent of locale

r275234 addressed sort automatically converting 8-bit locales to UTF-8
by using "LANG=C sort", but LC_ALL overrides LANG if set, so the issue
may still be present depending on the user's environment. Use LC_ALL=C
instead.

Reported by: tests.reproducible-builds.org
Reviewed by: bapt
MFC after: 1 week
Sponsored by: The Linux Foundation / Core Infrastructure Initiative
Differential Revision: https://reviews.freebsd.org/D9765