pandoc: Enable distribution to fix master eval

[srhb]

Motivation for this change

pandoc_2_6 has hydraPlatforms = stdenv.lib.platforms.none so I think the hackage2nix generator (or some other component) needs some more information so that master evals and thus grahamcofborg don't fail on nix-instantiate pkgs/top-level/release.nix -A pandoc.x86_64-linux

This is a workaround to keep the spice flowing.

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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[srhb]

@peti I can't figure out where the platforms get set to none for this version specifically, can you help with that for a more permanent fix? :)

[srhb]

Oh, it's exactly because it isn't in the default set.

[vcunat]

Seems more than safe enough, for a temporary fix.