pull: Display 'hash guix' hint when the new and old 'guix' are different.

Fixes <https://bugs.gnu.org/35601>.
Reported by Diego Nicola Barbato <dnbarbato@posteo.de>.

Previously we were calling (which "guix") after the new profile had been
built and symlinked, so in most cases the hint would not be triggered,
even though it should have been triggered.

* guix/scripts/pull.scm (build-and-install)[guix-command]: New variable.
Compare PROFILE/bin/guix against GUIX-COMMAND.

guix/scripts/pull.scm

@@ -203,6 +203,10 @@ true, display what would be built without actually building it."
   (define update-profile
     (store-lift build-and-use-profile))
 
+  (define guix-command
+    ;; The 'guix' command before we've built the new profile.
+    (which "guix"))
+
   (mlet %store-monad ((manifest (channel-instances->manifest instances)))
     (mbegin %store-monad
       (update-profile profile manifest
@@ -211,17 +215,18 @@ true, display what would be built without actually building it."
       (munless dry-run?
         (return (newline))
         (return (display-profile-news profile #:concise? #t))
-        (match (which "guix")
-          (#f (return #f))
-          (str
-           (let ((new (map (cut string-append <> "/bin/guix")
-                           (list (user-friendly-profile profile)
-                                 profile))))
-             (unless (member str new)
-               (display-hint (format #f (G_ "After setting @code{PATH}, run
+        (if guix-command
+            (let ((new (map (cut string-append <> "/bin/guix")
+                            (list (user-friendly-profile profile)
+                                  profile))))
+              ;; Is the 'guix' command previously in $PATH the same as the new
+              ;; one?  If the answer is "no", then suggest 'hash guix'.
+              (unless (member guix-command new)
+                (display-hint (format #f (G_ "After setting @code{PATH}, run
 @command{hash guix} to make sure your shell refers to @file{~a}.")
-                                     (first new))))
-             (return #f))))))))
+                                      (first new))))
+              (return #f))
+            (return #f))))))
 
 (define (honor-lets-encrypt-certificates! store)
   "Tell Guile-Git to use the Let's Encrypt certificates."