[1.3.0-RC4] no caching in crossbuild

[OlegYch]

steps

sbt version: 1.3.0-RC4
build.sbt

lazy val a = project.in(file(".")).settings(
  crossScalaVersions := List("2.12.9", "2.13.0", "2.11.12"),
)

src/main/scala/a.scala

object A {
  def a = 1
}

problem

running +compile recompiles the file even if it didn't change
compile works fine

expectation

+compile caches compilation results

notes

works fine with RC3

[eatkins]

Thanks for the report! Should be fixed by #4987.

[eed3si9n]

Merged #4987