matrix-synapse-plugins.matrix-http-rendezvous-synapse: fix maturin hook use

pkgs/servers/matrix-synapse/plugins/rendezvous.nix

···
     maturinBuildHook
   ]);
 
-  preBuild = ''
-    cd synapse
-  '';
+  buildAndTestSubdir = "synapse";
 
-  postBuild = ''
-    cd ..
-  '';
 
   pythonImportsCheck = [ "matrix_http_rendezvous_synapse" ];