Don't use deprecated ScalaInstance constructor

sbt/zinc@9397b6a#diff-ea135f2f26f43e40ff045089da221e1e
introduced a new constructor for ScalaInstance and deprecated the old
one. We use the new one but don't call it with the proper arguments. It
shouldn't matter though as the enw constructor was introduced to fix an
issue with `sbt run` that only occurs when `(fork in run) := false`.

Author: allanrenucci

project/Build.scala

@@ -249,6 +249,9 @@ object Build {
     // Compile using the non-bootstrapped and non-published dotty
     managedScalaInstance := false,
     scalaInstance := {
+      import sbt.internal.inc.ScalaInstance
+      import sbt.internal.inc.classpath.ClasspathUtilities
+
       val updateReport = update.value
       var libraryJar = packageBin.in(`dotty-library`, Compile).value
       var compilerJar = packageBin.in(`dotty-compiler`, Compile).value
@@ -270,8 +273,18 @@ object Build {
 
       val allJars = libraryJar :: compilerJar :: otherDependencies.toList
       val classLoader = state.value.classLoaderCache(allJars)
-      new sbt.internal.inc.ScalaInstance(scalaVersion.value,
-        classLoader, libraryJar, compilerJar, allJars.toArray, None)
+      new ScalaInstance(
+        scalaVersion.value,
+        classLoader,
+        ClasspathUtilities.rootLoader, // FIXME: Should be a class loader which only includes the dotty-lib
+                                       // See: https://github.com/sbt/zinc/commit/9397b6aaf94ac3cfab386e3abd11c0ef9c2ceaff#diff-ea135f2f26f43e40ff045089da221e1e
+                                       // Should not matter, as it addresses an issue with `sbt run` that
+                                       // only occur when `(fork in run) := false`
+        libraryJar,
+        compilerJar,
+        allJars.toArray,
+        None
+      )
     }
   )