Replace relative path with variable prefix

Author: ejona86

buildscripts/kokoro/linux_artifacts.sh

@@ -36,4 +36,4 @@ cp -r "$LOCAL_MVN_TEMP"/* "$MVN_ARTIFACT_DIR"/
 
 # for aarch64 platform
 sudo apt-get install -y g++-aarch64-linux-gnu
-SKIP_TESTS=true ARCH=aarch_64 ./buildscripts/kokoro/unix.sh
+SKIP_TESTS=true ARCH=aarch_64 "$GRPC_JAVA_DIR"/buildscripts/kokoro/unix.sh