pass-secret-service: Add meta.mainProgram

While this package had its name updated to reflect nix package naming
style, the binary's name is still `pass_secret_service`. Set the
`meta.mainProgram` attribute to allow for use of `lib.getExe`.

pkgs/applications/misc/pass-secret-service/default.nix

@@ -71,6 +71,7 @@ python3.pkgs.buildPythonApplication rec {
     homepage = "https://github.com/mdellweg/pass_secret_service/";
     license = lib.licenses.gpl3Only;
     platforms = lib.platforms.all;
+    mainProgram = "pass_secret_service";
     maintainers = with lib.maintainers; [ jluttine aidalgol ];
   };
 }