Revert "[makexpi] Run $VALIDATE in bg"

This reverts commit e0c5cad821460dab95c24ea51a490e2a13c0be9f.

Putting validate in the background means that when there is a failure and we
call `die`, the `exit 1` gets run in a subshell and so error status does not get
returned to the caller of makexpi.sh (i.e. test.sh).

Fixes #671