Emit Kotlin build reports (#4180)

Need a baseline to see how much time we save with [K2](#4170)

https://kotlinlang.org/docs/gradle-compilation-and-caches.html#build-reports

Author: rli

gradle.properties

@@ -22,3 +22,6 @@ org.gradle.jvmargs=-Xmx2048m
 
 # prefer non-enterprise variant of test-retry
 systemProp.gradle.enterprise.testretry.enabled=false
+
+# outputs to build/reports/kotlin-build
+kotlin.build.report.output=file