[CI] Nightly workflows tests failing since March 6 due to new unused_features lint on Nightly

[x0rw]

Issue:

While reviewing recent Actions, I noticed that nightly.yml test workflows have been failing since March 6: https://github.com/rustfoundation/safety-critical-rust-coding-guidelines/actions/workflows/nightly.yml

After investigating, I found that PR #152164 – Lint unused features which was merged on March 5th right after our last successful run, this PR introduced linting for unused #![feature(...)] directives. This causes warnings (treated as errors in our CI) for a test example because it declares a features like #![feature(test)] but did not actually use it.

Solutions:

• Change the failing test(tests/rust-examples/test-examples.rst:114) to use the declared feature.
  OR
• Set the fail_on_warnings = true to false in rust_examples_config.toml to allow warnings.