feat(tvix/store): use tonic-build for protos

While prost-build already exposes protobuf message types as structs, we
actually need tonic-build too, to be able to get traits for all the RPC
services defined in the proto files.

Change-Id: I7f4c08454bf0d280d577975c7cdae13ccc2d933b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7320
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/proto.rs

@@ -1,6 +1,6 @@
 use prost::Message;
 
-include!(concat!(env!("OUT_DIR"), "/tvix.store.v1.rs"));
+tonic::include_proto!("tvix.store.v1");
 
 impl Directory {
     // The size of a directory is the number of all regular and symlink elements,