chore(CI): Cache well-known-type protos with protoc binary

[arjan-bal]

Background: In #2665, the well-known-type protos were included in the output of the protoc-gen-rust-grpc crate, but CI only cached protoc and protoc-gen-rust-grpc.

Changes in this PR:

• Caches the included protos to easily reference them in the upcoming Google creds example (examples(grpc): Add example for Google Cloud Platform client #2680).
• Removes the redundant protoc installation via the taiki-e/install-action@protoc action, since prost-build now works with the cached binary.
• Ensures the protoc plugin is rebuilt whenever the workflow file changes, even if the source code remains untouched.

[ejona86]

Ah, so you were looking at this as well. I just sent out #2685

I think with my change it'd be okay to not include .github/workflows/CI.yml in the cache key, since there is so little logic in CI.yml with my change. I didn't notice/remove taiki-e/install-action@protoc. Is that installed protoc earlier in the PATH, so we're only using the cached protoc-gen-rust-grpc today?

[arjan-bal]

Ah, so you were looking at this as well. I just sent out #2685

I think with my change it'd be okay to not include .github/workflows/CI.yml in the cache key, since there is so little logic in CI.yml with my change. I didn't notice/remove taiki-e/install-action@protoc. Is that installed protoc earlier in the PATH, so we're only using the cached protoc-gen-rust-grpc today?

I had asked Doug to remove it in an earlier PR, but tonic's prost codegen relied on well-known type protos that were missing in the compiled protoc. Now that we're including those, we don't need the install action.

[arjan-bal]

Closing in favour of #2685.