[Gradle, JS] Fix gradle <5.1 incompatibility

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/ir/KotlinJsIrTargetConfigurator.kt

@@ -52,7 +52,8 @@ open class KotlinJsIrTargetConfigurator(kotlinPluginVersion: String) :
 
     override fun createJarTasks(target: KotlinJsIrTarget): Pair<String, Zip> {
         val (_, task) = super.createJarTasks(target)
-        task.archiveExtension.set(KLIB_TYPE)
+        // not archiveExtension because it is since Gradle 5.1 only
+        task.extension = KLIB_TYPE
 
         return KLIB_TYPE to task
     }