Added --path-to-built-config option to nixos-rebuild.sh to allow for building the system config manually, and then switching to it.

Motivation for this change

I wanted to experiment with using nix build to build my system image, but did not have a way to pass in the built system to nixos-rebuild without it redoing the work of the build.

Things done

Added --path-to-built-config option to nixos-rebuild.sh. Does not build the system image when executing either boot or switch, instead uses the supplied path.

• 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.