ci: set ARTIFACTS_DIR to KOKORO_ARTIFACTS_DIR in build script for Kokoro attestation (#407)

Change-Id: If5be41f46e95e030818956d786d14992b18a27d2

Co-authored-by: Anshul Patni <anshulpatni@google.com>

Author: AnshulPatni

.kokoro/build.sh

@@ -115,7 +115,7 @@ mvn clean deploy -B \
 # ==============================================================================
 # 4. Copy artifacts to 'artifacts/' folder for Kokoro Attestation Generation
 # ==============================================================================
-ARTIFACTS_DIR="${REPO_DIR}/artifacts"
+ARTIFACTS_DIR="${KOKORO_ARTIFACTS_DIR}/artifacts"
 mkdir -p "${ARTIFACTS_DIR}"
 
 # Copy target jars and poms (excluding test jars) to be captured by build.cfg