[K/N][build] Composite build with build.tools-conventions plugin

Adds convention plugin instead of sources inclusion
inside the buildSrc, effectively reducing the amount
of the code to be compiled in buildSrc.

Merge-request: KT-MR-8898

kotlin-native/klib/build.gradle

@@ -6,13 +6,13 @@ import org.jetbrains.kotlin.UtilsKt
 
 buildscript {
     apply from: "$rootDir/kotlin-native/gradle/kotlinGradlePlugin.gradle"
-
-    ext.useCustomDist = UtilsKt.getUseCustomDist(project)
 }
 
 apply plugin: 'kotlin'
 apply plugin: 'konan'
 
+ext.useCustomDist = UtilsKt.getUseCustomDist(project)
+
 konanArtifacts {
     def testFiles = fileTree('src/test/testData') {
         include "*.kt"