Use JVM toolchain for buildSrc module.

Before this change buildSrc module was compiled with current user JDK.
For example on CI it is JDK 1.8. But when the user is using JDK
different from 1.8, buildSrc compilation misses remote build cache. This
may lead to full project recomplilation on the user side.

buildSrc/build.gradle.kts

@@ -91,6 +91,12 @@ repositories {
     }
 }
 
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(8))
+    }
+}
+
 val generateCompilerVersion by tasks.registering(VersionGenerator::class) {
     kotlinNativeVersionInResources=true
     defaultVersionFileLocation()
@@ -189,11 +195,6 @@ samWithReceiver {
 fun Project.`samWithReceiver`(configure: org.jetbrains.kotlin.samWithReceiver.gradle.SamWithReceiverExtension.() -> Unit): Unit =
         extensions.configure("samWithReceiver", configure)
 
-java {
-    sourceCompatibility = JavaVersion.VERSION_1_8
-    targetCompatibility = JavaVersion.VERSION_1_8
-}
-
 tasks.withType<KotlinCompile>().configureEach {
     kotlinOptions.allWarningsAsErrors = true
     kotlinOptions.freeCompilerArgs += listOf(