fix ArtifactExists failure for bucket with large number of entries

Change-Id: I5a2eedb828831cbd9c87c73e383ae131e657bc7c
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/44270
Cloud-Build: GCB Service account <228075978874@cloudbuild.gserviceaccount.com>
Reviewed-by: Meena Shanmugam <meenashanmugam@google.com>
Tested-by: Arnav Kansal <rnv@google.com>
diff --git a/src/pkg/cos/artifacts.go b/src/pkg/cos/artifacts.go
index 639ee0e..16592ac 100644
--- a/src/pkg/cos/artifacts.go
+++ b/src/pkg/cos/artifacts.go
@@ -134,7 +134,7 @@
 func (d *GCSDownloader) ArtifactExists(artifactPath string) (bool, error) {
 	var objects []string
 	var err error
-	if objects, err = utils.ListGCSBucket(d.gcsDownloadBucket, d.gcsDownloadPrefix); err != nil {
+	if objects, err = utils.ListGCSBucket(d.gcsDownloadBucket, filepath.Join(d.gcsDownloadPrefix, artifactPath)); err != nil {
 		return false, errors.Wrap(err, "failed to find artifact")
 	}
 	for _, object := range objects {