plan and install should detect if nix is already installed

[rapenne-s]

That would be nice to detect if nix was already installed when trying install or plan, I suppose there code is there because the uninstall process already do that.

Here are the results of install and plan after a successful installation

$ sudo ./harmonic install
Error: 
   0: Error executing action
   1: Placing channel configuration
   2: Creating file
   3: File exists `/root/.nix-channels`

Location:
   src/cli/subcommand/install.rs:77

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SPANTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

   0: harmonic::cli::subcommand::install::execute
      at src/cli/subcommand/install.rs:43
   1: harmonic::cli::execute
      at src/cli/mod.rs:29

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

and

$ sudo ./harmonic plan
Error: 
   0: Error executing action
   1: Placing channel configuration
   2: Creating file
   3: File exists `/root/.nix-channels`

Location:
   src/cli/subcommand/plan.rs:58

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SPANTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

   0: harmonic::cli::subcommand::plan::execute with channels=nixpkgs https://nixos.org/channels/nixpkgs-unstable daemon_user_count=32 no_modify_profile=false
      at src/cli/subcommand/plan.rs:24
   1: harmonic::cli::execute
      at src/cli/mod.rs:29

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.