postgresql16Packages.pg_safeupdate: fix build on darwin

pkgs/servers/sql/postgresql/ext/pg_safeupdate.nix

···
   };
 
   installPhase = ''
-    mkdir -p $out/bin # for buildEnv, see https://github.com/NixOS/nixpkgs/issues/22653
-    install -D safeupdate.so -t $out/lib
+    install -D safeupdate${postgresql.dlSuffix} -t $out/lib
   '';
 
   meta = with lib; {