Linux

Now that we bundle genie_python as a pip package it's not really clear that we need explicit linux builds of the whole distribution any more - instead people should be able to install the relevant libraries (e.g. `genie_python`) via pip into a `.venv` created from system python.

We should document how to do that.

Or if I'm wrong and we _do_ need a full distribution build, then re-add the linux build scripts from https://github.com/ISISComputingGroup/genie_python_archive/tree/master/package_builder