clippy: make Cc/Rust toolchains optional

[lummax]

In certain situations (cross compilation with go) we might not have Cc/Rust toolchains that are available for the transitioned to execution/target platform.

For the linting case with clippy it is acceptable to just skip these targets.

See: https://github.com/lummax/rules_rust_clippy_go_cross_reproducer

[krasimirgg]

Looks good! Could you update this resolving the merge conflicts.

[UebelAndre]

The example confuses me. Do you have any toolchains registered at all? Doesn't clippy use of the --sysroot flag and require a valid sysroot?

[lummax]

The example confuses me. Do you have any toolchains registered at all? Doesn't clippy use of the --sysroot flag and require a valid sysroot?

--toolchain_resolution_debug=.* prints:

      ToolchainResolution:   Selected @@bazel_tools~cc_configure_extension~local_config_cc//:cc-compiler-k8 to run on execution platform @@platforms//host:host
[...]
      ToolchainResolution:   Selected @@rules_go~~go_sdk~go_default_sdk//:go_linux_amd64-impl to run on execution platform @@platforms//host:host
[...]
      ToolchainResolution:   Selected @@rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__nightly_tools//:rust_toolchain to run on execution platform @@platforms//host:host