Don't require tty for install-missing-packages

[bapt]

Summary

Fixes https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=267416

[patmaddox]

This is my first attempt at a code contribution to FreeBSD... hopefully I'm going about it the right way.

Anyway, I came across an issue when running make install-missing-packages on Cirrus CI, which has no TTY. This little change fixes it.

I admittedly don't know what the consequences of this change are. It looks like the -o flag was added in https://reviews.freebsd.org/D23795

Maybe it needs to have some sort of check to see if there's a TTY, and conditionally add the -o?

@0mp @mat @tobik care to weigh in?

[bapt]

the -o is on purpose to preserve interractive prompt, if you want to prevent that, then you need to make this conditionnal to BATCH: if BATCH then xargs pkg install -yA if not xargs -o pkg install -A

[bapt]

I have pushed a proper fix.