fetchurl: add passthru

[FRidh]

so that we could add e.g. the version attribute.

Motivation for this change

There have been discussions in the past about adding a version parameter to stdenv.mkDerivation. In my opinion, version is an attribute of the source, and therefore I propose having the possibility to add it to the source, using passthru.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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