Add CI Workflows and Test Coverage for toolchains_rust

Background
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Currently, the toolchains_rust repository does not include basic CI workflows for code formatting, copyright checks or automated cross-platform build tests. Adding these will improve code quality, ensure licensing compliance, and help guarantee that the toolchains work as expected for all supported platforms.

Feature Request
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I propose the following enhancements to the repository:

1. Code Formatting Check (Buildifier):

Add a GitHub Actions workflow (or equivalent CI) to automatically check and enforce Bazel file formatting using [buildifier](https://github.com/bazelbuild/buildtools/tree/master/buildifier).

2. Copyright/License Checks (cr-checker):

Integrate a workflow step to run [cr-checker]) l for copyright/license validation.

3. Cross-Platform Toolchain Testing:

Add a test/ subfolder to the repo with a simple Rust project (or set of smoke tests).

Add a CI workflow to build these tests using the defined toolchain(s) for all supported platforms.