I am attempting to prepend to the PYTHONPATH variable on slave nodes using the plugin. If I simply try to set the variable (Build Environment->Inject environment variables to the build process->Properties Content) like so:
PYTHONPATH=${WORKSPACE}/ref/dist/python

it does work. However, if I try to prepend to the variable like:
PYTHONPATH=${WORKSPACE}/ref/dist/python:$PYTHONPATH
or
PYTHONPATH=${WORKSPACE}/ref/dist/python:${PYTHONPATH}

I am met with the following error if the slave node hasn't already defined PYTHONPATH:
[EnvInject] - Executing scripts and injecting environment variables after the SCM step.
[EnvInject] - Injecting as environment variables the properties content
PYTHONPATH=${WORKSPACE}/ref/dist/python:${PYTHONPATH}

[EnvInject] - Variables injected successfully.
[EnvInject] - Unset unresolved 'PYTHONPATH' variable.

Then on my first build step, if I echo PYTHONPATH it's empty. It seems like merely referencing the variable in the value breaks down if it's not already defined. However, I can't assume that a slave node doesn't already have it set so I don't want to completely overwrite it.