Fix rules_rust target selection

[martijneken]

Actual Behavior

$ CARGO_BAZEL_REPIN=1 bazelisk build //samples/example:plugin_rust.wasm
ERROR: Target //samples/example:plugin_rust.wasm is incompatible and cannot be built, but was explicitly requested.
Dependency chain:
    //samples/example:plugin_rust.wasm
    //samples/example:_wasm_plugin_rust_wasm
    @crate_index//:hmac
    @crate_index__hmac-0.12.1//:hmac
    @crate_index__digest-0.10.7//:digest
    @crate_index__block-buffer-0.10.4//:block_buffer
    @crate_index__generic-array-0.14.7//:generic_array
    @crate_index__generic-array-0.14.7//:build_script_build
    @crate_index__generic-array-0.14.7//:generic-array_build_script
    @crate_index__version_check-0.9.4//:version_check   <-- target platform (@local_config_platform//:host) didn't satisfy constraint @platforms//:incompatible

All the generated Cargo targets have the same problem:

    target_compatible_with = select({
        "@rules_rust//rust/platform:wasm32-wasi": [],
        "//conditions:default": ["@platforms//:incompatible"],
    }),

Somehow we are not setting target platform correctly? We should be, via this rust_binary transition:
https://github.com/proxy-wasm/proxy-wasm-cpp-host/blob/master/bazel/wasm.bzl

Steps to Reproduce the Problem

1. Update proxy-wasm-cpp-host, which picks up feature: target_compatible_with added to CommonAttrs bazelbuild/rules_rust#1976
2. Build a Rust plugin, see build error

Workaround: set generate_target_compatible_with = False

[martijneken]

Somehow we are not setting target platform correctly? We should be, via this rust_binary transition

This might be the crux of the issue. Because we're using the ProxyWasm transition for rust_binary targets, perhaps the overall build (and thus crates_repository) does not have a wasm platform set.

Then again, providing the platform at the command line does not fix this:
bazelisk build --platforms="@rules_rust//rust/platform:wasi" //...

And the transition explicitly sets the platform:
https://github.com/proxy-wasm/proxy-wasm-cpp-host/blob/9be9637b5d595483ddba218eb09879818a5e0ffc/bazel/wasm.bzl#L22-L25

[martijneken]

Workaround applied in #36

[martijneken]

I bet the wasi_rust_binary_rule would work (it explicitly gets a platform attribute in the transition), but the rust_binary() rule here does not have a platform set (hence target platform @local_config_platform//:host)