This option can be set to disable installer tools like nixos-rebuild,
nixos-install, and nixos-generate-config (as well as more). This is
nice when a system is not expected to be rebuild or reconfigure itself
such as in a stateless PXE setup, as well as other embedded scenarios.
Note, that the system can still be updated, but it must either get
nixos-rebuild from another source, or, for embedded systems, be
upgraded by another machine like:

nix copy "$system" --to "ssh://root@" && ssh "root@"
"nix-env -p /nix/var/nix/profiles/system --set $system && $system/bin/switch-to-configuration switch".

Along with other options, this allows removing Perl from a closure.
For example:

{
boot.enableContainers = false;
environment.defaultPackages = [];
system.disableInstallerTools = true;
}

should not include Perl.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.