When running VIAME pipelines, pass VIAME_INSTALL env variable to pipeline calls

[mattdawkins]

Is your feature request related to a problem? If so, Please describe.

I want to import common VIAME routines in the trained detector.pipes when a user trains a model to simplify some of the .pipes that get generated and add different tracker modules to them.

Describe the solution you'd like

Pass in an environment variable called VIAME_INSTALL pointing to the root VIAME installation on the machine

Additional context

I'll add a example pipeline file to the example in the near future