add post build step instead of relying on upstream extensions

Author: aajtodd

builder.json

@@ -4,6 +4,9 @@
     "build_steps": [
         "./gradlew -x test -x allTests -x jvmTest build"
     ],
+    "post_build_steps": [
+        "./gradlew publishToMavenLocal"
+    ],
     "test_steps": [
         "./gradlew test allTests"
     ],