respect bb.edn adjacent to invoked file

[borkdude]

I'm considering the following change to bb:

~/my_project/bb.edn <- has medley in :deps 
~/my_project/script.bb <- uses medley.core, has #!/usr/bin/env bb shebang

/tmp $ ;; not inside ~/my_project
/tmp $ export PATH=$PATH:~/my_project
/tmp $ script.bb ;; works, uses ~/my_project/bb.edn as the "config" file
```

This lets you more easily create system-wide scripts, without using bbin