fix: use compatible prost/tonic crate versions for default toolchain

[kormide]

The tonic/prost versions used in the default toolchain don't work together, causing errors like:

error[E0599]: no associated item named `GRPC_STATUS` found for struct `Status` in the current scope
   --> bazel-out/k8-fastbuild/bin/external/googleapis+/google/devtools/build/v1/build_proto.lib.rs:932:48
    |
932 | ...                   tonic::Status::GRPC_STATUS,
    |                                      ^^^^^^^^^^^ associated item not found in `Status`

People can define their own rust_prost_toolchain, but the default set ought to still work.

Tested via a patch to rules_rs from a consuming module that calls rust_prost_library.

[kormide]

fyi @dzbarsky

[dzbarsky]

@kormide can you check if #59 works? I didn't succeed in getting my existing guinea pigs to verify it :)

[kormide]

@kormide can you check if #59 works? I didn't succeed in getting my existing guinea pigs to verify it :)

Oh I didn't see your PR. Tested with a local_path_override and yes, it works!

[dzbarsky]

Cool! I merged that and will cut new releases tomorrow after I get the bot account sorted out