fetchTarball: Use "source" as the default name
fetchTarball: Use "source" as the default name

This ensures that it produces the same output as fetchgit:

  $ nix eval --raw '(builtins.fetchgit https://github.com/NixOS/patchelf.git)'
  /nix/store/ghigrkw02l440g8vfxa9wj4c3zpfmw99-source

  $ nix eval --raw '(fetchTarball https://github.com/NixOS/patchelf/archive/master.tar.gz)'
  /nix/store/ghigrkw02l440g8vfxa9wj4c3zpfmw99-source

ehmry/nix,layus/nix,ehmry/nix,rycee/nix,NixOS/nix,zimbatm/nix,NixOS/nix,dtzWill/nix,dezgeg/nix,pikajude/nix,rycee/nix,peti/nix,rycee/nix,shlevy/nix,shlevy/nix,shlevy/nix,dtzWill/nix,ehmry/nix,ehmry/nix,layus/nix,peti/nix,pikajude/nix,layus/nix,NixOS/nix,peti/nix,peti/nix,rycee/nix,pikajude/nix,rycee/nix,dezgeg/nix,NixOS/nix,shlevy/nix,zimbatm/nix,ehmry/nix,zimbatm/nix,layus/nix,dezgeg/nix,shlevy/nix,zimbatm/nix,layus/nix,vcunat/nix,ehmry/nix,dtzWill/nix,dtzWill/nix,dezgeg/nix,peti/nix,vcunat/nix,vcunat/nix,vcunat/nix,dezgeg/nix,dtzWill/nix,NixOS/nix