Switch the order in which the version-specific attributes are combined
with args. Otherwise, if args such as 'branch' are overridden by an
overlay or other derivation, the new value is replaced with the
original when the sets are combined with //.

Motivation for this change

Overriding e.g. ffmpeg_4 in an overlay currently doesn't work, because the way the attributes provided by the override are combined with the attributes in pkgs/development/libraries/ffmpeg/4.nix results in the original attributes shadowing the overrides.

This change should have no effect on existing packages, unless they have "unused" attributes currently being shadowed.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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 nix-review --run "nix-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.

Notify maintainers

cc @