Allow `stdenv` of firefox addons to be overriden

Michael Hoangrequested to merge
Enzime/nur-expressions:override-stdenv into master

By using lib.makeOverridable, all the arguments for Firefox addons can be overridden using .override.

One use case occurs when using unfree Firefox addons with flakes:

firefox-addons.onepassword-password-manager.override { stdenv = pkgs.stdenv.override { config.allowUnfree = true; }; }

Otherwise NIXPKGS_ALLOW_UNFREE=1 and --impure would be required on every command line invocation.

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