nix: strip PYTHONPATH from env when wrapping aws-auth
nix: strip PYTHONPATH from env when wrapping aws-auth

this should isolate aws-auth's python env from the enclosing session's
python env, particularly a problem when one is py2 and the other py3

alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws