mdformat: Support more than 1 plugin at a time

pkgs/by-name/md/mdformat/package.nix

···
         };
       }
       ''
-        buildPythonPath $plugins
+        buildPythonPath "$plugins"
         makeWrapper ${lib.getExe python.pkgs.mdformat} $out/bin/mdformat \
           --suffix PYTHONPATH : "$program_PYTHONPATH"
       '';