Fix scala-js/scala-js#2202: Prevent concurrent use of linker in sbt

We use sbt's tag mechanism to prevent concurrent execution.

The test failed 60 consecutive times before the change and succeeded 60
consecutive times after it on a 2 core machine.

sbt-plugin-test/build.sbt

@@ -27,7 +27,11 @@ lazy val noDOM = project.settings(baseSettings: _*).
     scalaJSOutputWrapper := (
         "// Scala.js - noDOM sbt test\n//\n// Compiled with Scala.js\n",
         "// End of Scala.js generated script")
-  )
+  ).
+  /* This hopefully exposes concurrent uses of the linker. If it fails/gets
+   * flaky, there is a bug somewhere - #2202
+   */
+  settings(inConfig(Compile)(run <<= run.dependsOn(fastOptJS, loadedJSEnv)): _*)
 
 lazy val withDOM = project.settings(baseSettings: _*).
   enablePlugins(ScalaJSPlugin).