xsane: build with snapshot version of the sane backends

The last release of that code is ancient. Not sure what these SANE guys
are doing. Anyway, the current Git snapshot works just fine for me.

pkgs/top-level/all-packages.nix

@@ -9067,6 +9067,7 @@ let
 
   xsane = callPackage ../misc/xsane {
     libpng = libpng12;
+    saneBackends = saneBackendsSnapshot;
   };
 
   yafc = callPackage ../applications/networking/yafc { };