fix(resource): use meltano_bin param

[tomtomau]

I've been having a lot of trouble trying to get meltano running in a dagster code location as a package
i.e.

load_from:
  - python_package:
      package_name: dag_meltano
      executable_path: dag/meltano/.venv/bin/python

I kept butting up against #53, which seems like meltano isn't installed in the virtualenv, but I can confirm it definitely is in my package's virtualenv 🤔

I wanted to specify the meltano bin manually and noticed this was already here, but did nothing? So I think this is somewhat productive and I'm hoping I can use this to specify the path directly to the bin