Fix minibuffer-help-form for lexical binding

* lisp/simple.el (set-variable): Substitute var into
minibuffer-help-form.
* lisp/cus-edit.el (custom-prompt-variable): Likewise.

lisp/cus-edit.el

@@ -917,7 +917,7 @@ the current value of the variable, otherwise `symbol-value' is used.
 If optional COMMENT argument is non-nil, also prompt for a comment and return
 it as the third element in the list."
   (let* ((var (read-variable prompt-var))
-	 (minibuffer-help-form '(describe-variable var))
+	 (minibuffer-help-form `(describe-variable ',var))
 	 (val
 	  (let ((prop (get var 'variable-interactive))
 		(type (get var 'custom-type))

lisp/simple.el

@@ -7930,7 +7930,7 @@ With a prefix argument, set VARIABLE to VALUE buffer-locally."
 		   (read-variable (format "Set variable (default %s): " default-var)
 				  default-var)
 		 (read-variable "Set variable: ")))
-	  (minibuffer-help-form '(describe-variable var))
+	  (minibuffer-help-form `(describe-variable ',var))
 	  (prop (get var 'variable-interactive))
           (obsolete (car (get var 'byte-obsolete-variable)))
 	  (prompt (format "Set %s %s to value: " var