Revert "loader/lua: Remove compat shim for loader.lua_path"

This reverts commit ab97d42addae97a389c6f22d6bba62ed954bb8e7.

There's too many people in the field with FreeBSD 12.0 loader.efi that
stubbed their toe on upgrading to 14.1 since they'd not updated
loader.efi. While we sort out that mess, add back this workaround. Can
revisit after 14.2 maybe.

MFC After: 3 days
Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D45882