prober: run all probes once on initial registration.

Turns out, it's annoying to have to wait the entire interval
before getting any monitorable data, especially for very long
interval probes like hourly/daily checks.

Signed-off-by: David Anderson <danderson@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f