gnu: ungoogled-chromium: Use 'search-auxiliary-file'.

Fixes a regression introduced in
076e825dc5 whereby "guix build
ungoogled-chromium" would fail with:

  canonicalize-path: No such file or directory: "aux-files/chromium/master-preferences.json"

* gnu/packages/chromium.scm (ungoogled-chromium)[arguments]: Use
'search-auxiliary-file' for "master-preferences.json".  For some reason,
on Guile 3.0.8, 'local-file' would have its (current-source-directory)
expand to #f.

gnu/packages/chromium.scm

@@ -757,7 +757,9 @@
                      (resources      (string-append lib "/resources"))
                      (preferences
                       ;; This file contains defaults for new user profiles.
-                      #$(local-file "aux-files/chromium/master-preferences.json"))
+                      #$(local-file
+                         (search-auxiliary-file
+                          "chromium/master-preferences.json")))
                      (gtk+           (assoc-ref inputs "gtk+"))
                      (xdg-utils      (assoc-ref inputs "xdg-utils")))