Switch MatrixProject to use config-customWorkspace

This change factors the current "or" gated optionalBlock into two optionalBlock's.
1) is now defined by config-customWorkspace
2) is now defined in a compatible way to recent 1.585 core changes.

This way of factoring the change allows it to work with older cores, with different definitions of "config-customWorkspace", or 1.585+ cores.


This is tracked by: https://issues.jenkins-ci.org/browse/JENKINS-25221