postgresql: running initdb from command line now works (#65309)

The issue was only with NixOS service, `postgresql` installed through
`nix-env` was not affected.

Fixes https://github.com/NixOS/nixpkgs/issues/23655

nixos/modules/services/databases/postgresql.nix

@@ -251,6 +251,10 @@ in
 
     environment.systemPackages = [ postgresql ];
 
+    environment.pathsToLink = [
+     "/share/postgresql"
+    ];
+
     systemd.services.postgresql =
       { description = "PostgreSQL Server";