DH_PIP_EXTRA_ARGS doesn't work

[dekkers]

According to the documentation, extra arguments to pip can be specified using DH_PIP_EXTRA_ARGS or --extra-pip-arg. DH_PIP_EXTRA_ARGS doesn't seem to be working. Reading the code I see DH_PIP_EXTRA_ARGS being used in the perl code, but it is the python code that calls pip and that only supports --extra-pip-arg.