Improve --profile help text

[edolstra]

Describe the bug

Currently the help text for MixProfile just says

      · --profile path
        
        The profile to operate on.

which is generic and not very helpful. It should say something about what the command does with the profile, but MixProfile doesn't know that.

Steps To Reproduce

Expected behavior

A clear and concise description of what you expected to happen.

nix-env --version output

Additional context

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