The relative paths option for protoc generators doesn't work well when
it comes to dependencies. This simplifies the code generation to avoid
using `go generate` and to use one global command for protoc generation.
This is similar to https://github.com/docker/buildx/pull/2713 since the
same problems with code generation occur here too.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>