limit build process output, to avoid bombing Isabelle/Scala process by ill-behaved jobs (e.g. Containers in AFP/9025435b29cf);

Isabelle-hg: http://isabelle.in.tum.de/repos/isabelle@51962 016cb7d8f297cd6a15be16bd249e24b53e1ec3ff
recovered informative progress from 016cb7d8f297;

Isabelle-hg: http://isabelle.in.tum.de/repos/isabelle@51966 0e18eee8c2c274591330c4aac0870df03d74db03