Merge build and runtime deps install commands

[maximlt]

This had two effects:

• it slowed down the install
• could cause Python to be updated in between the two calls, leading to doit reporting a failure but without an explicit error

This PR merges the build and runtime deps install commands into a single command. It might lead to slightly different solve results though.

[maximlt]

Arf no that doesn't quite work actually!

The function _get_setup_metadata tries first to open setup.cfg but will usually fail because the HoloViz projects use setup.py, and then tries to import setup.py to get the install_requires dependencies. In doing so it may encounter some imports needed to run setup.py, like importing pyct, which is one of the build dependencies that hasn't yet been installed.

This could be revisited the day the projects all move to setup.cfg or pyproject.toml.