fix: remove outdated uses of user-emacs-directory

Doom changes user-emacs-directory (see 46844b5 for explanation), so
doom-emacs-dir should be used to refer to your $EMACSDIR instead.

Fix: #6530
Ref: 46844b55de

core/core-modules.el

@@ -377,7 +377,7 @@ This value is cached. If REFRESH-P, then don't use the cached value."
            (or (and (stringp arg)
                     (not (file-name-absolute-p arg))
                     (ignore-errors (dir!)))
-               user-emacs-directory)))
+               doom-emacs-dir)))
       (funcall fn label arg recursed)))
 
   ;; Adds two keywords to `use-package' to expand its lazy-loading capabilities: