Add /usr/local/bin to autoenv search path (#5481)

The current list of directories to search for autoenv on misses the default location on Ubuntu systems if you just do a normal `pip install autoenv` - [it will place](https://github.com/kennethreitz/autoenv/blob/master/setup.py#L16) `activate.sh` in `/usr/local/bin` unless you manually override the `--prefix` or something.

The `/usr/local/opt/autoenv` is fine for macOS/homebrew installations but it would be nice not to have to manually patch on Linux :)