Motivation for this change

The desktop file bundled with nextcloud calls the icon nextcloud, but the
icons are named Nextcloud and so desktop environments won't find it.

Note that upstream uses the opposite workaround in their appimage creation script (renaming an icon file to match what the desktop file expects): https://github.com/nextcloud/client_theming/blob/d0a46ea2f04eb9fe1b99d4cedfb595bc7664d27f/linux/AppImage/build-appimage.sh#L61

I'm using the capture group to find matching lines because the translations generate lines like Icon[ja_JP]=nextcloud and those need to be changed too.

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.