librewolf: allow addon sideloading

LibreWolf's build configuration[1] already disables addon sideloading,
so this isn't a change in behavior.

[1] https://codeberg.org/librewolf/source/src/commit/e5762e9d5badf50bbc72f578d01111ed6e92dd74/assets/mozconfig.new#L3

pkgs/applications/networking/browsers/librewolf/default.nix

···
   version = librewolf-src.packageVersion;
   src = librewolf-src.firefox;
   requireSigning = false;
+  allowAddonSideload = true;
   inherit (librewolf-src) extraConfigureFlags extraPatches extraPostPatch extraPassthru;
 
   meta = {