gtk3: temporarily disable stalled theme genaration

[hedning]

Motivation for this change

gtk ships with themes, but supports regenerating them with sassc, unfortunately
libsass-3.6.2 has bug which makes this stall forever.

Note I'm not sure how many other themes (sass/libsass#3006) will break with the newer libsass, so might be better to downgrade that?

cc #71221

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 @

[hedning]

Ah, libsass was reverted: ddee207

[FRidh]

Ah, I just reverted libsass bump instead
ddee207

[hedning]

Yeah saw 😆, probably better that way, as the newer libsass would probably break a bunch of other themes.