nixos-rebuild: support -vv, -vvv, -vvvv, and -vvvvv

I ran into a case where I needed to use `-vvvv` on `nixos-rebuild`, and got tripped up for a while before I read the source and realized I needed to use `-v -v -v -v`.  This patch makes `nixos-rebuild` support the other single-character argument combinations.  Tested manually.