[boost] Passing variables to bjam when run by regression.py?

Hello, I need to pass some variables (PYTHON_...) to bjam in order to compile the python tests. When invoking bjam directly, boost.build picks up those variables from the environment automatically. When running the regression tests with regression.py, the environment variables seem to be ignored, because I get a warning that my python installation is not found. How do I pass environment variables to boost.build when running it from regression.py? Or is there some other way to pass arguments to bjam when it is run by regression.py? TIA, Markus