Truncate doctor-negative-savings-tasks to a sensible limit to avoid a warning

[valeraz]

Getting this warning when running some of our builds with v0.9.2.

WARNING: 1 build scan custom value was not captured:
 - Maximum custom value value length (100,000) exceeded: doctor-negative-savings-tasks

Discussed offline that, despite the fact that the limit varies based on the backend install, it might make sense to truncate output to that limit anyway.