Use a precompiled `protoc` toolchain

[phlax]

we have just updated the Envoy repo to make use of precompiled protoc which speeds a load of things up

recompiling the go protos i immediately noticed that i was compiling protoc

one option is using protobuf from rules_proto which makes use of the precompiled bins but this then ties you to their version of protobuf

another option is to patch protobuf, this is what Envoy does