bug: mongodb fails to start when using non-default port

The reason is that the port is not being used by the [probe command](https://github.com/juspay/services-flake/blob/8bc6dff1c0d82842b28e3906ac4645a3c3a49dbe/nix/services/mongodb.nix#L84) (as well as the init command, however due to failing probe it never gets executed anyway)

I've got [an implementation](https://github.com/adrian-gierakowski/services-flake/commit/fe1df6753f6ded7dffb5b6a9bf95be3455d2474c) of a mongodb services I've been using privately, from which I would like to port missing features (like ability to run a single-member replica set, which is needed to use [change streams](https://www.mongodb.com/docs/manual/changeStreams/)) to this repo. I can fix this bug while I'm at it.