broken header streamer after v3.48.0

The ansi-timestamps experiment is enabled in v3.48.0 by default. The header line begins with `_bk;t=1698038886128~~~ `, the header streamer for header regex https://github.com/buildkite/agent/blob/24c562342c8dd3944b16bc6b87b501e304490e67/agent/header_times_streamer.go#L167C25-L167C74 broken, no header streamer anymore. The regex needs to be updated to ` headerRE = regexp.MustCompile(`^(_bk;t=\d+(?:---|~~~|\+\+\+)\s(.+))$`) ` to fix the header streamer.