Reproducible Issue with rust toolchain in external pigweed repo

[michael-christen]

Reproduce by running:

bazel build //tools:lib --toolchain_resolution_debug="@@rules_rust~//rust:toolchain_type"

Note that default fails to work because of the clang CPP toolchain. Registering the toolchain in WORKSPACE gets it selected, but then ends up with:

INFO: ToolchainResolution: Target platform @@platforms//host:host: Selected execution platform @@platforms//host:host, type @@rules_rust~//rust:toolchain_type -> toolchain @@pigweed~//pw_toolchain/rust:host_rust_toolchain_linux_x86_64_stable_rust_toolchain, type @@bazel_tools//tools/cpp:toolchain_type -> toolchain @@pigweed~//pw_toolchain/host_clang:host_toolchain
ERROR: no such package '@@[unknown repo 'rust_toolchain_host_linux_x86_64' requested from @@pigweed~]//': The repository '@@[unknown repo 'rust_toolchain_host_linux_x86_64' requested from @@pigweed~]' could not be resolved: No repository visible as '@rust_toolchain_host_linux_x86_64' from repository '@@pigweed~'
ERROR: /home/mchristen/.local/tmp/bazel/_bazel_mchristen/fe66862a52f25f8003207d6b27dc05b6/external/pigweed~/pw_toolchain/rust/BUILD.bazel:22:34: no such package '@@[unknown repo 'rust_toolchain_host_linux_x86_64' requested from @@pigweed~]//': The repository '@@[unknown repo 'rust_toolchain_host_linux_x86_64' requested from @@pigweed~]' could not be resolved: No repository visible as '@rust_toolchain_host_linux_x86_64' from repository '@@pigweed~' and referenced by '@@pigweed~//pw_toolchain/rust:host_rust_toolchain_linux_x86_64_stable_rust_toolchain'
ERROR: Analysis of target '//tools:lib' failed; build aborted: Analysis failed

[michael-christen]

No need to merge / deal with @bsodmike, I was just using this as an example to help show an issue with rust toolchains in repos that use pigweed