Default LEFT to /run/current-system

I often find myself diffing my current system and some other version (e.g. `nix-diff /run/current-system /nix/store/4jivfbzaj6zp9m7zy3y1zd8n4p95cgkp-nixos-system-francis-25.05.806796.92c2e04a4755`). It'd be nice if I could omit that first argument. Is that a good default?

Start at the nix-diff command-line entry point and compare the documented invocation with the example using /run/current-system. Check how the two positional arguments are currently handled; done means omitting the first argument compares against /run/current-system while the existing two-argument form still works.

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