xdg-user-dirs: check for existing symlink

When a non-directory, such as a file or a dead symlink, already exists,
mkdir -p fails with "cannot create directory ‘...’: File exists".

This is a problem when, for example, a symlink points to a directory on
a filesystem that isn't mounted yet.

modules/misc/xdg-user-dirs.nix

@@ -137,7 +137,8 @@ in {
 
     home.activation.createXdgUserDirectories = mkIf cfg.createDirectories (let
       directoriesList = attrValues directories;
-      mkdir = (dir: ''run mkdir -p $VERBOSE_ARG "${dir}"'');
+      mkdir =
+        (dir: ''[[ -L "${dir}" ]] || run mkdir -p $VERBOSE_ARG "${dir}"'');
     in lib.hm.dag.entryAfter [ "linkGeneration" ]
     (strings.concatMapStringsSep "\n" mkdir directoriesList));
   };