Access javaTask.classpath property only in doFirst because otherwise it doesn't contain classes added after apply of kotlin plugin

#KT-7277 Fixed
 #KT-9899 Fixed

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/AnnotationProcessingManager.kt

@@ -46,7 +46,9 @@ fun Project.initKapt(
         val stubsDir = File(getBuildDir(), "tmp/kapt/$variantName/classFileStubs")
         kotlinTask.extensions.extraProperties.set("kaptStubsDir", stubsDir)
 
-        javaTask.classpath += files(stubsDir)
+        javaTask.doFirst {
+            javaTask.classpath += files(stubsDir)
+        }
 
         kotlinTask.doFirst {
             kotlinAfterJavaTask.source(kotlinTask.source)

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPlugin.kt

@@ -134,8 +134,9 @@ class Kotlin2JvmSourceSetProcessor(
 
         if (javaTask != null) {
             javaTask.dependsOn(kotlinTaskName)
-            val javacClassPath = javaTask.getClasspath() + project.files(kotlinDestinationDir);
+            javaTask.doFirst {
-            javaTask.setClasspath(javacClassPath)
+                javaTask.classpath += project.files(kotlinDestinationDir)
+            }
         }
 
         val kotlinAnnotationProcessingDep = cachedKotlinAnnotationProcessingDep ?: run {