Use ubuntu2004 for Envoy
Use ubuntu2004 for Envoy

gcc version is too old on ubuntu1804, which caused https://github.com/envoyproxy/envoy/issues/19196
bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration