gnu: poppler: Always use the same 'cairo-sans-poppler' variant.

* gnu/packages/gtk.scm (cairo-sans-poppler): New variable.
* gnu/packages/pdf.scm (poppler)[inputs]: Use it instead of
locally-created variant.

gnu/packages/gtk.scm

@@ -173,6 +173,12 @@ affine transformation (scale, rotation, shear, etc.).")
    (license license:lgpl2.1) ; or Mozilla Public License 1.1
    (home-page "https://cairographics.org/")))
 
+(define-public cairo-sans-poppler
+  ;; Variant used to break the dependency cycle between Poppler and Cairo.
+  (package/inherit cairo
+    (inputs (alist-delete "poppler" (package-inputs cairo)))
+    (properties `((hidden? . #t)))))
+
 (define-public cairo-xcb
   (package
     (inherit cairo)

gnu/packages/pdf.scm

@@ -191,9 +191,7 @@ please install the @code{flyer-composer-gui} package.")))
 
              ;; To build poppler-glib (as needed by Evince), we need Cairo and
              ;; GLib.  But of course, that Cairo must not depend on Poppler.
-             ("cairo" ,(package (inherit cairo)
-                         (inputs (alist-delete "poppler"
-                                               (package-inputs cairo)))))))
+             ("cairo" ,cairo-sans-poppler)))
    (propagated-inputs
     ;; As per poppler-cairo and poppler-glib.pc.
     ;; XXX: Ideally we'd propagate Cairo too, but that would require a