Remove os.environ.copy in Dataset.map
I thought deepcopy on `os.environ` is unsafe (see [this](https://stackoverflow.com/questions/13142972/using-copy-deepcopy-on-os-environ-in-python-appears-broken)), but I can't replicate the behavior described in the linked SO thread.

Closing this one because #2119 has a much cleaner approach.
Replace `os.environ.copy` with in-place modification
Fixes #2115 