Use JDK 8 as JAVA_HOME for kapt tests

* kapt tests launch scripts in a child process relied on JAVA_HOME
* tests fail under JDK 11 due to illegal reflexive access

plugins/kapt3/kapt3-cli/build.gradle.kts

@@ -28,4 +28,8 @@ projectTest {
     useJUnitPlatform()
     workingDir = rootDir
     dependsOn(":dist")
+    val jdkHome = project.getToolchainJdkHomeFor(JdkMajorVersion.JDK_1_8)
+    doFirst {
+        environment("JAVA_HOME", jdkHome.get())
+    }
 }