Don't call `getenv` in side threads

[eqy]

Calling getenv on side threads is dangerous as it can potentially segfault if the main thread is in the middle of setting environment variables: https://github.com/pytorch/pytorch/issues/134596

This PR only calls getenv on init rather than in Config() which can be called from multiple threads.

CC @malfet @nWEIdia @ptrblck @tinglvv