cloud build: use files from csi-release-tools

cmd/csi-snapshotter/Dockerfile

@@ -1,6 +1,7 @@
 FROM gcr.io/distroless/static:latest
 LABEL maintainers="Kubernetes Authors"
 LABEL description="CSI External Snapshotter Sidecar"
+ARG binary=./bin/csi-snapshotter
 
-COPY ./bin/csi-snapshotter csi-snapshotter
+COPY ${binary} csi-snapshotter
 ENTRYPOINT ["/csi-snapshotter"]