Specify extra packages in conda CI script

@inducer

build-py-project.sh reads env variable "EXTRA_INSTALL", and install additional packages. I don't find similar code in build-py-project-within-miniconda.sh. I am trying to configure CI for distributed boxtree on github. I don't want to add mpi4py to requirements.txt, but still want the CI to install it. Does the current conda CI script support this?