Do not restart the agent if `POST /update` call should do nothing

[Xayton]

When the agent processes the POST /update command, it will check if the latest version is newer than the agent version. If it's not, the command should not do anything. Instead it always restarts the agent.

• It should also download the full executable only when it's not on the latest version, instead of always doing it.
• If possible, the user agent used when downloading the updates should be changed from the default Go-http-client/2.0 to something like Arduino-Cloud-Agent/1.5.0.

https://github.com/arduino/arduino-create-agent/blob/main/update.go#L37-L49
The code above should be improved.