Feature: Allow setting the project path in an environment variable

My repo structure looks something like this:
```
/root
└─ /sqlmesh
   └─ config.py
```

And I usually call sqlmesh from the root with `sqlmesh -p sqlmesh info`.
It would be lovely if I was given the option to set the project path in an environment variable, so I only need to run `sqlmesh info`.