Minor tweaks to fix grpc example

[chrisirhc]

Made some changes to #58 to get the example to work.

I'm still seeing the following error but I think it's unrelated and seems reproducible on main branch:

ERROR: toolchain type @@rules_rust_prost~//:toolchain_type resolved to target //tools/toolchains:prost_toolchain_impl, but that target does not provide ToolchainInfo
ERROR: Analysis of target '//proto:greeter_rust_proto' failed; build aborted

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