Revert "Don't use copy.deepcopy with os.environ"

As it turns out larger chunks of Craft are dependent on the environment being filled in by this method despite what the code originally intended to happen.
Hopefully fixing the Path -> PATH will resolve the issue hit on MingW builds when signing.

This reverts commit 100afc1715688fc1a8d2a7e0c167151f6938d45b.