envoy: Filter out unhelpful warnings
envoy: Filter out unhelpful warnings

Filter out the annoying "warning: directory does not exist" warnings
when building protobufs. A preceding "INFO: From ProtoCompile" lines
still remain, but there are only a handful of them, and they provide
an indication that something was filtered out.

Fixes: #4703
Signed-off-by: Jarno Rajahalme <0f1ab0ac7dffd9db21aa539af2fd4bb04abc3ad4@covalent.io>

cilium/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,scanf/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,scanf/cilium,cilium-team/cilium,cilium-team/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,scanf/cilium,michi-covalent/cilium,tklauser/cilium,scanf/cilium,scanf/cilium,scanf/cilium,cilium/cilium,scanf/cilium,tgraf/cilium