Making BUILD_DOKKA=true also override the version number of the Dokka dependency

Otherwise, BUILD_DOKKA=true may have no effect if Dokka's version differs from what AndroidX expects

Bug: 148450617
Test: BUILD_DOKKA=true ./gradlew dokkaKotlinTipOfTreeDocs
Change-Id: I30af0f270017d527611cef4a67acd36c59971564
diff --git a/buildSrc/build.gradle b/buildSrc/build.gradle
index 50ada7d..4561ed1 100644
--- a/buildSrc/build.gradle
+++ b/buildSrc/build.gradle
@@ -1,7 +1,7 @@
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 buildscript {
-    def supportRootFolder = project.projectDir.getParentFile()
+    project.ext.supportRootFolder = project.projectDir.getParentFile()
     repositories {
         maven {
             url "${supportRootFolder}/../../prebuilts/androidx/external"