To use a locally built Scala version, define the scalaHome setting, which is of type Option[File]. This Scala version will only be used for the build and not for sbt, which will still use the version it was compiled against.
scalaHome := Some(file("/path/to/scala"))
sbt reuses the class loader for the local Scala version. If you recompile your local Scala version and you are using sbt interactively, run
to use the new compilation results.