This is required for the shims to handle `#!/usr/bin/env python3` in a
shebang, just like `python` is handled currently: it will set
`PYENV_DIR` to the root of the invoked script, which is required for a
`.python-version` script to get picked up from there.
This was rejected for rbenv, where it does not make much sense
(https://github.com/sstephenson/rbenv/pull/735).
Ref: https://github.com/yyuu/pyenv/pull/368#issuecomment-102806837
