multi-user toggle init

modules/shell/exercism.nix

@@ -7,11 +7,16 @@
 {
   options.my.shell.exercism.enable = lib.mkEnableOption "Exercism coding practice platform";
   config = lib.mkIf config.my.shell.exercism.enable {
-    users.users.jawz.packages = builtins.attrValues {
+    users.users = let
+      users = if lib.isString config.my.toggleUsers.shell then [ config.my.toggleUsers.shell ] else config.my.toggleUsers.shell;
+      packages = builtins.attrValues {
       inherit (pkgs)
         exercism # learn to code
         bats # testing system, required by Exercism
         ;
-    };
+      };
+    in lib.mkMerge (map (user: {
+      ${user}.packages = packages;
+    }) users);
   };
 }