Resolve scala from maven, don't build it.

Simplify pr validation for the IDE by always resolving from maven.
Two changes were needed to enable this:
  - don't compute `FULL_SCALA_VERSION`, just take `SCALA_VERSION`
  - honor `MAVEN_ARGS` from the environment (don't overwrite them).
Pass prRepoUrl as a Java property.

The sbt launcher used for dbuild can't read environment vars (or I don't know how to make it),
so must pass it the Java way.
Make defaults overridable. Quiet down checkNeeded.