Two bsb with socket running makes the second one's rebuild detection fail

Repro:

- Create 2 projects, each with `bsb -make-world -w -ws _ ` (aka default ws port).
- Start both.
- Change a file in the second project.
- See that the second bsb doesn't rebuild.

This should probably error the same way we're currently erroring when there are more than one `bsb` running for the same project.