buildd/functions.sh: use mutt instead of nail again.

In git commit bbc4b9db068e7ed536161680c84fbed328ddb3af
back in january 2010 we had to switch from mutt to nail
because of an annoying behaviour change in attachment
handling. Now having mutt 1.5.20 in the relevant Debian
suites available let's turn back to mutt, because
nail sucks at attachment handling even more.