Fix bin override (#2)
Fix bin override (#2)

When the user uses a `"bin": "some/path"` override in the `.arclinter`
config, this extension can no longer spawn the binary to get the version
by using the default binary name.

See https://secure.phabricator.com/book/phabricator/article/arcanist_lint/ in the
`Locating Binaries and Interpreters` section and the docstring for
`ArcanistExternalLinter::getExecutableCommand()`:

```
Get the composed executable command, including the interpreter and binary but without flags or paths. This can be used to execute --version commands.
```
material-foundation/arc-tslint