nix: put treefmt in devshell
treefmt is a useful to be able to access directly for some formatters like `jj fix`. Expose it in the devshell. Test plan: - Used with `jj fix` on a large branch. It worked.
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treefmt is a useful to be able to access directly for some formatters like `jj fix`. Expose it in the devshell. Test plan: - Used with `jj fix` on a large branch. It worked.