Allow setting PDM_PREFER_BINARY in pyproject.toml

[2e0byo]

Currently to make pdm prefer binaries (a sensible default when working teams across different archs and machines) you have to use an env var. It would be nice to be able to set this in pyproject.toml so it happens automatically.

I'm guessing by the use of an env var that passing the state around is non-trivial. What about allowing setting any env var in pyproject.toml and having pdm set it? (extending what's already possible for tool.pdm.scripts)

If this is already possible and I've missed it, sorry!