pantheon.file-roller-contract: fix version, add passthru.updateScript

Upstream doesn't tag release, we use unstableGitUpdater.

pkgs/desktops/pantheon/desktop/file-roller-contract/default.nix

@@ -1,13 +1,14 @@
 { lib
 , stdenv
 , fetchFromGitHub
+, unstableGitUpdater
 , substituteAll
 , file-roller
 }:
 
 stdenv.mkDerivation rec {
   pname = "file-roller-contract";
-  version = "unstable-2021-02-23";
+  version = "unstable-2021-02-22";
 
   src = fetchFromGitHub {
     owner = "elementary";
@@ -36,6 +37,12 @@ stdenv.mkDerivation rec {
     runHook postInstall
   '';
 
+  passthru = {
+    updateScript = unstableGitUpdater {
+      url = "https://github.com/elementary/file-roller-contract.git";
+    };
+  };
+
   meta = with lib; {
     description = "Contractor extension for File Roller";
     homepage = "https://github.com/elementary/file-roller-contract";