refactor: (if|when)-let -> (if|when)-let*

With the former macros' future in the air (and likely to be targeted in
future, potentially breaking changes), I'll deal with this now than have
it bite me later.

Ref: https://lists.gnu.org/archive/html/emacs-devel/2024-10/msg00637.html

lisp/doom.el

@@ -191,9 +191,9 @@
 ;;; Fix $HOME on Windows
 ;; $HOME isn't normally defined on Windows, but many unix tools expect it.
 (when doom--system-windows-p
-  (when-let (realhome
-             (and (null (getenv-internal "HOME"))
-                  (getenv "USERPROFILE")))
+  (when-let* ((realhome
+               (and (null (getenv-internal "HOME"))
+                    (getenv "USERPROFILE"))))
     (setenv "HOME" realhome)
     (setq abbreviated-home-dir nil)))
 
@@ -221,7 +221,7 @@
   "The time it took, in seconds (as a float), for Doom Emacs to start up.")
 
 (defconst doom-profile
-  (if-let (profile (getenv-internal "DOOMPROFILE"))
+  (if-let* ((profile (getenv-internal "DOOMPROFILE")))
       (save-match-data
         (if (string-match "^\\([^@]+\\)@\\(.+\\)$" profile)
             (cons (match-string 1 profile)
@@ -244,7 +244,7 @@
 
 (defvar doom-user-dir
   (expand-file-name
-   (if-let (doomdir (getenv-internal "DOOMDIR"))
+   (if-let* ((doomdir (getenv-internal "DOOMDIR")))
        (file-name-as-directory doomdir)
      (or (let ((xdgdir
                 (file-name-concat
@@ -280,7 +280,7 @@ slash.")
 
 ;; DEPRECATED: .local will be removed entirely in 3.0
 (defvar doom-local-dir
-  (if-let (localdir (getenv-internal "DOOMLOCALDIR"))
+  (if-let* ((localdir (getenv-internal "DOOMLOCALDIR")))
       (expand-file-name (file-name-as-directory localdir))
     (expand-file-name ".local/" doom-emacs-dir))
   "Root directory for local storage.