add env variable for CB (com-lihaoyi#209)

To be able to override the version of scala for CB, we need to add it
via environment variables.

Author: hamzaremmal

build.mill

@@ -11,8 +11,9 @@ import mill.scalalib.publish.{Developer, License, PomSettings, VersionControl}
 import mill.util.VcsVersion
 import com.github.lolgab.mill.mima._
 
+val communityBuildDottyVersion = sys.props.get("dottyVersion").toList
 val scalaNextVersion = sys.props.get("scalaNextVersion")
-val scalaVersions = List("2.12.20", "2.13.15", "3.3.4") ++ scalaNextVersion
+val scalaVersions = List("2.12.20", "2.13.15", "3.3.4") ++ scalaNextVersion ++ communityBuildDottyVersion
 val scalaNativeVer = "0.5.6"
 
 trait MimaCheck extends Mima {