Add buildKotlinVersion Gradle property
Add buildKotlinVersion Gradle property

to quickly run the Gradle build using the plugin of a version
different than the embedded Kotlin

Signed-off-by: Paul Merlin <a027184a55211cd23e3f3094f1fdc728df5e0500@gradle.com>

blindpirate/gradle,gradle/gradle,gradle/gradle,blindpirate/gradle,blindpirate/gradle,blindpirate/gradle,gradle/gradle,gradle/gradle,gradle/gradle,blindpirate/gradle,gradle/gradle,gradle/gradle,gradle/gradle,gradle/gradle,blindpirate/gradle,blindpirate/gradle,blindpirate/gradle,blindpirate/gradle,gradle/gradle,blindpirate/gradle