thunderbird-bin: Set explicit binaryName after betterbird changes

The betterbird package introduced the binaryName variable, because it
uses a different executable filename. During review the required changes
to thunderbird-bin were missed.

pkgs/applications/networking/mailreaders/thunderbird-bin/default.nix

···
     baseUrl = "http://archive.mozilla.org/pub/thunderbird/releases/";
   };
 
+  passthru = {
+    binaryName = "thunderbird";
+  };
+
   meta = with lib; {
     changelog = "https://www.thunderbird.net/en-US/thunderbird/${version}/releasenotes/";
     description = "Mozilla Thunderbird, a full-featured email client (binary package)";