为什么不能在build.gradle.kts中使用`const val`

I'd like to define a version constant in guild.gradle.kts file so that it can be used even in the plugins block. The plugins block requires restricted syntax:

«plugin version» must be constant, literal, strings

遵循限制,我尝试定义一个版本常量:

const val kotlinVersion = "1.3.72"

plugins {
    java
    kotlin("jvm") version kotlinVersion
}

但是这失败并显示消息

Const 'val' are only allowed on top level or in objects

even though the declaration seem to meet all const requirements. Why cannot be const val used in build.gradle.kts?