ci: add missing dependency for TTY prereq

ci: add missing dependency for TTY prereq

In "t/lib-terminal.sh", we declare a lazy prerequisite for tests that
require a TTY. The prerequisite uses a Perl script to figure out whether
we do have a usable TTY or not and thus implicitly depends on the PERL
prerequisite, as well. Furthermore though, the script requires another
dependency that is easy to miss, namely on the IO::Pty module. If that
module is not installed, then the script will exit early due to an
reason unrelated to missing TTYs.

This easily leads to missing test coverage. But most importantly, our CI
systems are missing this dependency and thus don't execute those tests
at all. Fix this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>