nixos-install: cleanups & improvements to run on non-NixOS systems

- Fix --no-bootloader which didn't do what it advertised
- Hardcode nixbld GID so that systems which do not have a nixbld user
  can still run nixos-install (only with --closure since they can't
  build anything)
- Cleanup: get rid of NIX_CONF_DIR(=/tmp)/nix.conf and pass arguments instead
- Cleanup: don't assume that the target system has '<nixpkgs/nixos>' or
  '<nixos-config>' to see if config.users.mutableUsers. Instead check if
  /var/setuid-wrappers/passwd is there

Installing NixOS now works from a Ubuntu host (using --closure).

nix-build -A tests.installer.simple '<nixpkgs/nixos/release.nix>' succeeds ✓