Avoid calling `std::env::temp_dir` on WASI

See #364. I'm not sure if we can reasonably default to "/tmp", but, in v4, we _CAN_: 1. Change `tempfile::env::temp_dir` to return a `Result` and/or `Option`. 2. Return some form of "temporary directory not set" _error_ from the tempfile constructors instead of panicking.