Partially-downloaded server jar's are not handled well at the moment, preventing container start

Hi,

I've had a couple of cases recently where I've had to start and then quickly stop a new minecraft server container.
The scripts have correctly started downloading the latest jar but the container was stopped before it had finished.
On restarting the container I get ` Error: Invalid or corrupt jarfile minecraft_server.1.20.6.jar` or similar, unsurprisingly.

I would make sense to me to first download into a temporary file with a modified name, then rename. On every start we could delete any partially-downloaded files matching the temporary pattern, and then continue to re-download if required.

I'm happy to do this work (I submitted a PR for the bedrock image a while back).

Would this be better to be done as part of your `mc-image-helper get` in that repo or in this one, perhaps in `start-utils.sh`?

Thanks for all your effort on this.

### Container logs

```
[init] Setting initial memory to 1G and max to 1G
[init] Starting the Minecraft server...
Error: Invalid or corrupt jarfile minecraft_server.1.20.6.jar
```

Start by reading start-utils.sh and checking the mc-image-helper get entry point mentioned in the issue to determine where the server jar download is handled. Reproduce a stopped download and restart, then verify that incomplete temporary files are cleaned up and a completed download is installed without producing an invalid jarfile.

Written by the indexing model from the issue text.