I set up a very simple job, with the only change from the default free-style job being checking the "Prepare environment for the run" and adding one line to the properties content of "QMAKEFEATURES=$WORKSPACE".

When I run the build, I get the messages:
[EnvInject] - Injecting as environment variables the properties content
QMAKEFEATURES=$WORKSPACE

[EnvInject] - Variables injected successfully.
[EnvInject] - Injecting contributions.
[EnvInject] - Unset unresolved 'QMAKEFEATURES' variable.