Motivation for this change

common-updater-script uses cmp command to check errors, but it cannot find cmp, so results in the following log file:

/nix/store/ys5l41pwfja3lyjaw9mf9ia9ivi8y8kl-common-updater-scripts/bin/.update-source-version-wrapped: line 55: cmp: command not found
/nix/store/ys5l41pwfja3lyjaw9mf9ia9ivi8y8kl-common-updater-scripts/bin/.update-source-version-wrapped: line 69: cmp: command not found
/nix/store/ys5l41pwfja3lyjaw9mf9ia9ivi8y8kl-common-updater-scripts/bin/.update-source-version-wrapped: line 86: cmp: command not found

This PR adds diffutils as a dependency to fix it.

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
  • Linux
• 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.

cc: @dezgeg