Make the shebang for `arc` refer to python2
Make the shebang for `arc` refer to python2

Summary:

When you try to run `arc` from within a python3 virtualenv, it fails, since our
wrapper script is written in python2.  However, since it has no external
dependencies, and everyone will have python2 installed for ka dev for the
forseeable future, we can just go ahead and explicitly run with python2.

Test Plan:
- `arc lint` from inside a python3 virtualenv, verify the command succeeds, where
  before it would fail.

Auditors: csilvers

Khan/arcanist,Khan/arcanist,Khan/arcanist,Khan/arcanist