Motivation for this change

The parsing regex for output hash is broken for the current nix in unstable channel.
This PR ports patches to fix it.

Behavior on current master:

$ nix run nixpkgs.nix-prefetch -c nix-prefetch hello.src
The fetcher will be called as follows:
> fetchurl {
>   sha256 = "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i";
>   url = "mirror://gnu/hello/hello-2.10.tar.gz";
> }

trying https://ftpmirror.gnu.org/hello/hello-2.10.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:00:01 --:--:--     0
100  708k  100  708k    0     0   111k      0  0:00:06  0:00:06 --:--:--  189k
hash mismatch in fixed-output derivation '/nix/store/3x7dwzq014bblazs7kq20p9hyzz0qh8g-hello-2.10.tar.gz':
  wanted: sha256:0000000000000000000000000000000000000000000000000000
  got:    sha256:0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i
error: build of '/nix/store/mhsd2ar52m4d0f2hmlkhbpj4rnc4kws0-hello-2.10.tar.gz.drv' failed

msteen/nix-prefetch#8 tried to fix the issue but closed in flavor of msteen/nix-prefetch#9, which is also closed and redo-ed in master (msteen/nix-prefetch@2722cda). However, it still has bug, which is finally fixed by msteen/nix-prefetch#12

I opened an issue to upstream about making a new release in msteen/nix-prefetch#13 but there's no response until now.
So I patch here to make it work at least.

@Mic92

Things done

• 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.