status: Distinguish 'package-cache' profile hook.

* guix/status.scm (hook-message): Handle 'package-cache'.

guix/status.scm

@@ -322,6 +322,8 @@ on."
      (G_ "building TeX Live configuration..."))
     ('manual-database
      (G_ "building database for manual pages..."))
+    ('package-cache                    ;package cache generated by 'guix pull'
+     (G_ "building package cache..."))
     (_ #f)))
 
 (define* (print-build-event event old-status status