grpcio-tools: Add riscv64 wheel build

[luhenry]

• Package: grpcio-tools
• Version: 1.83.1
• Source: https://github.com/grpc/grpc/tree/master/tools/distrib/python/grpcio_tools
• Docs: https://grpc.io

Compiles gRPC's protoc codegen plugin (grpc_tools._protoc_compiler) into a native extension used to generate Python/gRPC code from .proto files. Upstream publishes no riscv64 wheel.

Mirrors this repo's build-grpcio.yml - same grpc/grpc source tree, same cibuildwheel/ccache pattern; upstream builds these internally via Bazel/Kokoro, not a public GH Actions workflow.

Differs from upstream

• package-dir: tools/distrib/python/grpcio_tools, since the package lives nested in the monorepo rather than at its root like grpcio.
• Runs tools/distrib/python/make_grpcio_tools.py before the build - it stages grpc_root/third_party sources upstream's own install_all_python_modules.sh also depends on, with the bazel-query step it also attempts failing non-fatally (falls back to the deps list already committed for this tag).

Matrix: cp312/cp313/cp314 only, same as build-grpcio.yml - grpcio-tools shares python_version.py's supported range and declares no free-threaded support.

Testing

• python -m grpc_tools.protoc --help, matching upstream's own binary distribtest (test/distrib/python/test_packages.sh)
• grpc_tools/test/protoc_test.py, generating complicated_pb2 with a plain protoc invocation in place of the Bazel py_proto_library target it normally relies on

License: OK

CI confirms the build itself: all three interpreters compile and auditwheel-repair cleanly. grpcio-tools pins grpcio>=1.83.1; the registry only has up to 1.78.0 (1.83.1 is in open #836), so CIBW_TEST_ENVIRONMENT: PIP_ONLY_BINARY=:all: fails fast at wheel-install instead of falling back to an hours-long from-source grpcio build. Draft until #836 lands.