Allow shutil to copy into existing isolation directory

[christophert]

Fixes #1309 by adding dirs_exist_ok=True to the copytree call. Tested locally and the directory correctly populates with project directory files.

[Shrews]

We need to dig into the history of this. It seems there is a directory_isolation_cleanup configuration setting that doesn't appear to be exposed that is supposed to control cleaning up of the isolation directory. I'm not certain we should always just blindly overwrite it based on the existence of this setting.

[christophert]

It looks like that setting would control cleaning it up once the run was completed. This change allows copytree to copy into the newly made directory by tempfile.mkdtemp in line 135, not any existing project isolation directory created by previous runs.

[Shrews]

dirs_exist_ok added in Python 3.8, so this should be ok.