alsa-lib: provide update script

pkgs/by-name/al/alsa-lib/package.nix

···
   alsa-topology-conf,
   alsa-ucm-conf,
   testers,
+  directoryListingUpdater,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
···
     "dev"
   ];
 
-  passthru.tests.pkg-config = testers.testMetaPkgConfig finalAttrs.finalPackage;
+  passthru = {
+    tests.pkg-config = testers.testMetaPkgConfig finalAttrs.finalPackage;
+    updateScript = directoryListingUpdater {
+      url = "https://www.alsa-project.org/files/pub/lib/";
+    };
+  };
 
   meta = {
     homepage = "http://www.alsa-project.org/";