Use absolute path for logfile output

Co-authored-by: Sergii Tkachenko <[email protected]>

Author: murgatroid99

packages/grpc-js-xds/scripts/xds.sh

@@ -59,7 +59,7 @@ GRPC_NODE_TRACE=xds_client,xds_resolver,cds_balancer,eds_balancer,priority,weigh
     --gcp_suffix=$(date '+%s') \
     --verbose \
     ${XDS_V3_OPT-} \
-    --client_cmd="$(which node) --enable-source-maps --prof --logfile=grpc/reports/prof.log grpc-node/packages/grpc-js-xds/build/interop/xds-interop-client \
+    --client_cmd="$(which node) --enable-source-maps --prof --logfile=${KOKORO_ARTIFACTS_DIR}/grpc/reports/prof.log grpc-node/packages/grpc-js-xds/build/interop/xds-interop-client \
       --server=xds:///{server_uri} \
       --stats_port={stats_port} \
       --qps={qps} \