update to use source_dir which now references the current project source dir always

Author: aajtodd

builder.json

@@ -2,7 +2,7 @@
     "name": "aws-crt-kotlin",
     "packages": [],
     "variables": {
-        "gradlew": "{project_source_dir}/gradlew -p {project_source_dir}"
+        "gradlew": "{source_dir}/gradlew -p {source_dir}"
     },
     "build_steps": [
         "{gradlew} -x test -x allTests -x jvmTest build"