SYNAPSE_HTTP_PORT is not replaced in generated config file

This issue has been migrated from [#12396](https://github.com/matrix-org/synapse/issues/12396).

---

### Description

While config generation, the `SYNAPSE_HTTP_PORT` is not replaced. Consequently the port is always `8008`.

### Steps to reproduce

- Run the following command
```
docker run -it --rm \
--mount type=volume,src=synapse-data,dst=/data \
-e SYNAPSE_SERVER_NAME=testserver \
-e SYNAPSE_HTTP_PORT=1234 \
-e SYNAPSE_REPORT_STATS=no \
matrixdotorg/synapse:latest generate
```
- Check the generated `homeserver.yaml` file

### Version information

- **Homeserver**: -

If not matrix.org:

- **Version**: v1.55.2

- **Install method**: Kustomize

- **Platform**: Kubernetes 1.20+