roachprod: output start and end timestamp to ssh logs

We currently preserve ssh debug logs in the case of any failure, however there is no way to see what time the command actually ran. The timestamp encoded in the log file currently, only indicates when the command was initialised, not when it was executed.

There is no easy way to show timestamps alongside the debug log output. A process substitution solution worked in bash, but not zsh. Additionally, it relies on enriching the existing error output of the command, on which some parts of the code may already depend.[1]

A compromise would be to emit start and end timestamps to the ssh debug file, which operates outside of stdout/stderr.

[1] A subset of read accesses of `RunResultDetails.Stderr`

Jira issue: CRDB-26277