Possible namespace collision in pep517 code

[njsmith]

We create build directories keyed by the sdist hash. This is handy if we end up wanting both metadata and a wheel from the same sdist within the same run, because we can re-use the build directory. BUT, in principle we might end up building the same sdist for different platforms within a single run, and in that case they can't share the same directory. So we should include the platform in the directory key, or something like that.

(I guess we also re-select the build platform every time we call get_env_for_build? That seems suboptimal... probably we should end up getting the same result every time but still, maybe we should pick it once and store that somewhere to re-use.)