fish: enable manpage completion

This patch follows a similar patch[1] in nixpkgs. With this patch,
fish can complete manpages for programs installed through
home-manager, e.g., using home.packages.

[1]: https://github.com/NixOS/nixpkgs/pull/91794

modules/programs/fish.nix

@@ -285,6 +285,9 @@ in {
     {
       home.packages = [ cfg.package ];
 
+      # Support completion for `man` by building a cache for `apropos`.
+      programs.man.generateCaches = mkDefault true;
+
       xdg.dataFile."fish/home-manager_generated_completions".source = let
         # paths later in the list will overwrite those already linked
         destructiveSymlinkJoin = args_@{ name, paths, preferLocalBuild ? true