tmuxPlugins: re-add mkTmuxPlugin
tmuxPlugins: re-add mkTmuxPlugin

`mkTmuxPlugin` was removed from `tmuxPlugins` in #115718 while changing
the name from `mkDerivation`; this reintroduces it under the new name.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs