`protoc` is again logging unhelpful missing directory warnings

Since upgrading protobuf CI is spewing a lot of missing directory warnings

This implies (and/or) that (iiuc):

- ~CI is not using the precompiled `protoc`~
- ~CI is using a very old `protoc`~

EDIT: upstream regression

Otherwise protobuf has some regression to log these warnings, ~but i think above are more likely explanations~