Fix build failure when default features disabled

[jacobmichels]

Description

I want to leverage nix-installer as a rust library. I tried building the library without any features enabled and got compilation errors due due to usage of clap without the cli feature enabled (which I presume brings in clap).

This PR simply adds some checks to ensure the cli feature is enabled before calling anything from clap. Only two files needed to be modified before the code compiled without any features enabled.

Example error

error: cannot find attribute `clap` in this scope
   --> /Users/<username>/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nix-installer-0.23.0/src/planner/mod.rs:180:42
    |
180 |     #[cfg_attr(not(target_os = "linux"), clap(hide = true))]

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

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