frontend: don't queue build chroots with incomplete info
The use of outerjoin() was wrong, there should be join() from the
beginning (548a9a05e1f2fd1294a4b3d0e4b6691a85779fdf, and probably even
before).
That's because we can not process build_chroot without knowing what
Package we actually build and what CoprDir we build into. The
BuildChroot.build_id is naturally also required, but we have non-null
constraint there.
Fixes: #1321
Relates: #1269