rust, rustfmt: add conflicts_with because both install cargofmt and rustfmt

[chenrui333]

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Possible conflicting files are:
/opt/homebrew/bin/cargo-fmt -> /opt/homebrew/Cellar/rustfmt/1.5.2/bin/cargo-fmt
/opt/homebrew/bin/rustfmt -> /opt/homebrew/Cellar/rustfmt/1.5.2/bin/rustfmt

followup of #136514