Use patched protobuf with RUSTSEC-2019-0003 fix.

[jsirois]

A crates index patch was needed here to ensure both our crates and
transitive dependent crates saw the same rust-protobuf. Without this we
hit many errors like:

   Compiling bazel_protos v0.0.1 (/home/jsirois/dev/pantsbuild/jsirois-pants/src/rust/engine/process_execution/bazel_protos)
error[E0277]: the trait bound `gen::bytestream::ReadRequest: protobuf::core::Message` is not satisfied
  --> process_execution/bazel_protos/src/gen/bytestream_grpc.rs:23:42
   |
23 |     req_mar: ::grpcio::Marshaller { ser: ::grpcio::pb_ser, de: ::grpcio::pb_de },
   |                                          ^^^^^^^^^^^^^^^^ the trait `protobuf::core::Message` is not implemented for `gen::bytestream::ReadRequest`
   |
   = note: required by `grpcio::codec::pb_codec::ser`

Fixes #7760

[jsirois]

NB: We already were being floated up to 2.0.6 by cargo (see Cargo.lock) - I just edited these to make it more clear how the cargo index patch applies.

[Eric-Arellano]

I think we can kill this --ignore flag below now, right?

pants/build-support/bin/ci.sh

Lines 258 to 263 in 91a88ba

	# TODO(John Sirois): Kill --ignore RUSTSEC-2019-0003 when we can upgrade to an official released
	# version of protobuf with a fix.
	# See: https://github.com/pantsbuild/pants/issues/7760 for context.
	"${REPO_ROOT}/build-support/bin/native/cargo" audit \
	-f "${REPO_ROOT}/src/rust/engine/Cargo.lock" \
	--ignore RUSTSEC-2019-0003

While the TODO says to wait until the official patch is landed, with us now using Pants' version of protobuf I think we can clean that part up now. The only remaining cleanup once the patch lands would be to update Cargo.toml.

[jsirois]

I think we can kill this --ignore flag below now, right?

Nope. There is no remedy recognized by the database. We'd need to clone the database, edit RUSTSEC-2019-0003 and cron refreshes of the db.

Edit: They just committed remedies: https://github.com/RustSec/advisory-db/commits/master/crates/protobuf/RUSTSEC-2019-0003.toml however we'll need to upgrade through several minor versions to get there. I've filed #7771 to track that yak. I need to be moving on at the moment to other work.