Note that deepcopy doesn't work either, even though it looks like it does at the first glance (which is arguably worse since it's harder to notice):

Python 3.8.6 (default, Jun  4 2021, 05:16:01)
>>> import copy, os, subprocess
>>> env_copy = copy.deepcopy(os.environ)
>>> env_copy["TEST"] = "oh no"
>>> os.environ["TEST"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/fbcode/platform009/lib/python3.8/os.py", line 675, in __getitem__
    raise KeyError(key) from None
KeyError: 'TEST'
>>> subprocess.run("echo $TEST", shell=True, capture_output=True).stdout.decode()
'oh no\n'