use a conda env.yaml for installing packages on build

[raybellwaves]

Following https://tljh.jupyter.org/en/latest/topic/customizing-installer.html#installing-python-packages-in-the-user-environment which allows packages to me installed on the user env on building give uses a --user-requirements-txt-url flag. It would be nice to offer a conda alternative flag e.g. --user-environment-url

[raybellwaves]

This could also be done via the plugins https://tljh.jupyter.org/en/latest/topic/customizing-installer.html#installing-tljh-plugins