Fix regular nix-prefetch-* output handling
Fix regular nix-prefetch-* output handling

The #8 PR contained this like from #6 while it shouldn't, because the
latter PR was changing the output of `hashFromNixPrefetch` to include
the full `sha256 = "...";` line but this not used anymore in #8.

kamilchm/go2nix,kamilchm/go2nix