istio: proxy-postsubmit failed to build proxy

It seems the postsubmit failed since https://github.com/istio/proxy/pull/2310 with the following message:

Target //src/envoy:envoy_tar up-to-date:
  /home/bootstrap/.cache/bazel/_bazel_bootstrap/45ae47b4a0e12d1e81c831ece04d820d/execroot/__main__/bazel-out/k8-opt/bin/src/envoy/envoy_tar.tar.gz
INFO: Elapsed time: 939.158s, Critical Path: 213.68s
INFO: 2258 processes: 2258 processwrapper-sandbox.
INFO: Build completed successfully, 3056 total actions
INFO: Build completed successfully, 3056 total actions
+ BAZEL_TARGET=bazel-bin/src/envoy/envoy_tar.tar.gz
+ cp -f bazel-bin/src/envoy/envoy_tar.tar.gz envoy-symbol-00bd27fad4e78cf5d803f9b39010b71103eda183.tar.gz
cp: cannot stat 'bazel-bin/src/envoy/envoy_tar.tar.gz': No such file or directory

@utka Could you help take a look? This is blocking Istio from updating the proxy sha which is needed for https://github.com/istio/istio/issues/15631 for 1.3, thank you!