update rust-g to 3.9.0 and use formatted_timestamp for human_readable_timestamp

[Absolucy]

About The Pull Request

This updates rust-g to 3.9.0, and also improves some code related to logging a little:

• rustg_unix_timestamp is now a define instead of a proc
  • this is somewhat of a hot call, as it gets called every time a log entry is written, so this is prolly worth it imo
• /datum/log_holder/proc/human_readable_timestamp() now uses rustg_formatted_timestamp, instead of the current stupid BS with time2text and then adding fractional seconds by using splittext on the unix timestamp???
  • i've confirmed that the output of the old and new code are identical.

this is a port of tgstation/tgstation#91121, which itself was a partial port of #6540, my old PR here

Why It's Good For The Game

because using splittext on a timestamp to add a fractional part is stupid, when we can now just use rust-g to format it all in one go.

Changelog

No user-facing changes.