installers/tools: add system.disableInstallerTools option

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@<host>" && ssh "root@<host>"
  "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.

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