ci: add line length lint step using nightly cargo fmt

[antiguru]

rustfmt refuses to format files that have too long lines, which results in some files silently skipping formatting. Add a new CI lint step that runs cargo +nightly fmt -- --check --config error_on_line_overflow=true to catch overly long lines in Rust source files. This check is only available on nightly Rust.

[github-actions]

Pre-merge checklist

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• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
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