copy.deepcopy os.environ instead of copy
Fixes: https://github.com/huggingface/datasets/issues/2115

- bug fix: using envrion.copy() returns a dict.
- using deepcopy(environ) returns an `_environ` object
- Changing the datatype of the _environ object can break code, if subsequent libraries perform operations using apis exclusive to the environ object, like `environ.getenv()` for example.


Testing:

Tested the change on my terminal:

```
>>> import os
>>> x = deepcopy(os.environ)
>>> y = os.environ
>>> x is y
False
>>> isinstance(x, type(os.environ))
True
>>> z = os.environ.copy()
>>> isinstance(z, type(os.environ))
False
>>> isinstance(z, dict)
True
```