cea608tott: Don't zero-pad SRT indices but zero-pad hours to at least two digits

Zero-padding is not specified for the indices but all time components
need to be zero-padded (3 digits for fractional seconds, 2 digits for
everything else).
12 jobs for !704 with 0.8-backports in 12 minutes and 54 seconds (queued for 2 seconds)
merge request