Update scalastyle linter

Now uses the scalastyle binary before launching the jar with java.
The existing code doesn't use the interpreter; it sets the binary as `java` and [adds a flag](https://github.com/coursera/arcanist/blob/308413e4a2d150d42d5c500a25dc3ba82db8e46d/src/lint/linter/ArcanistScalastyleLinter.php#L47) for the Scalastyle JAR.

I think we should preserve or remove the existing behavior (recognizing a JAR