makechrootpkg: Split out chrootbuild into a function

Now syntax highlighting works properly! :D
arch-nspawn: Quiet systemd-nspawn again

systemd-nspawn always outputs some debug messages over stderr.
Both stdout and stderr from inside the chroot are sent through
a pty to stdout.
makechrootpkg: Suppress makepkg --verifysource stderr

Doing this should make makechrootpkg completely silent
on stderr again, unless an error occurs.