fetchurl: Don't force-override curl's gssSupport to on unnecessarily

[nh2]

• Based on fetchurl: Make it overridable using callPackage #66503, merge that first (simply because the changed lines are close together and may result in a conflict if done independently).

Motivation for this change

Bug #66499

The original intent in commit a1fec86 treewide: assemble all `fetchurlBoot` uses in overrides to `fetchurl` itself was to turn gssSupport off on Darwin, but the code actually also forced
it on on Linux.

This resulted in previous (e.g. overlays) .override { gssSupport = false; } being ignored (#66499).

This commit fixes it by just respecting the old value when it doesn't need to be forced to off.

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 @oxij @infinisil @matthewbauer @Ericson2314

[nh2]

• Based on fetchurl: Make it overridable using callPackage #66503, merge that first (simply because the changed lines are close together and may result in a conflict if done independently).

That was merged, I have rebased.

[nh2]

Just for clarification, the previous version of the code was equivalent to this:

       gssSupport =
        if stdenv.isDarwin || stdenv.hostPlatform.isWindows
          then false
          else true

and I've made it this:

       gssSupport =
        if stdenv.isDarwin || stdenv.hostPlatform.isWindows
          then false
          else old.gssSupport or true; # `? true` is the default

[oxij]

Yep, that's a bug in a1fec86. This LGTM.