ui: Avoid circularity with (guix gexp).

Fixes a regression introduced in 56b8210 ("guix build: Allow gexps to be
passed to '-e'.")

* guix/ui.scm (%guix-user-module): Wrap in 'delay'.
  (read/eval): Adjust accordingly.

guix/ui.scm

@@ -240,11 +240,13 @@ interpreted."
 
 (define %guix-user-module
   ;; Module in which user expressions are evaluated.
-  (let ((module (make-module)))
-    (beautify-user-module! module)
-    ;; Use (guix gexp) so that one can use #~ & co.
-    (module-use! module (resolve-interface '(guix gexp)))
-    module))
+  ;; Compute lazily to avoid circularity with (guix gexp).
+  (delay
+    (let ((module (make-module)))
+      (beautify-user-module! module)
+      ;; Use (guix gexp) so that one can use #~ & co.
+      (module-use! module (resolve-interface '(guix gexp)))
+      module)))
 
 (define (read/eval str)
   "Read and evaluate STR, raising an error if something goes wrong."
@@ -256,7 +258,7 @@ interpreted."
                         str args)))))
     (catch #t
       (lambda ()
-        (eval exp %guix-user-module))
+        (eval exp (force %guix-user-module)))
       (lambda args
         (leave (_ "failed to evaluate expression `~a': ~s~%")
                exp args)))))