bugfix: don't redundantly print ChildErrors (#17709)

A bug was introduced in #13100 where ChildErrors would be redundantly
printed when raised during a build. We should eventually revisit error
handling in builds and figure out what the right separation of
responsibilities is for distributed builds, but for now just skip
printing.

- [x] SpackErrors were designed to be printed by the forked process, not
      by the parent, so check if they've already been printed.
- [x] update tests