That script does a lot of work that boils down to "go to https://nixos.org/releases/nix/nix-1.7 and do the obvious"
Of course if thats not signed you still have a problem. Eternal chicken and egg issues. How do you install gpg to verify a gpg signature while verifying the gpg signature of gpg
Installation instructions for nix. Although I'm not sure that arbitrarily running shell scripts from the internet is a good thing. When I install an OS X app from the internet, at least it's signed.