--help fails in a confusing way if man isn't installed

[noamraph]

Describe the bug

This is related to #6186

I installed nix in a docker container, in order to play with it. The --help argument of nix-* commands fails, and it was quite difficult to understand how to fix it:

$ nix-env --help
error: command 'man nix-env' failed: No such file or directory

A suggestion: perhaps add a note saying something like "Showing help requires man installed. You can install it using your OS package manager".

Steps To Reproduce

1. Install nix in an ubuntu:22.04 docker container.
2. Run nix-env --help

Expected behavior

See the help

nix-env --version output

nix-env (Nix) 2.22.1

Additional context

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