Fix issues with `latest` symlinks

[redpanda-f]

Verify and fix the issue where-in latest symlinks are not pointing to the correct "run-id" in ~/.foc-devnet. Reported by @rvagg .

@rvagg can you provide me with reproducible steps if not too big of a hassle on when and how latest symlink breaks away from the right run-id?

[rvagg]

No I can't, but I encountered it more than once and I had to manually fix it to get my scripts to work that look at latest. Perhaps it's to do with overlapping starts? Does start check if there's a devnet already started before it goes ahead and updates this?
If nobody else sees this issue then we could close this but let's keep it open and collect instances. Next time I encounter this I'll try and collect more data.

[rvagg]

Experienced it again: cargo run -- build makes it happen. Every command does init_logging(run_id) and in there is update_latest_symlink(&run_dir), so every command you run gets latest updated. Maybe it should only happen for start?