Every Action of a job shared one trace.
Its root was the span of the job, which one Action announced in its main phase
and started in its post phase.
Thus each trace pointed at a span that did not exist until the job ended, and
that never existed at all when that post phase did not run.

Now each execution phase is the root of a trace of its own.
Three IDs join those traces, and nothing else does:
`detsys.invocation_id` for the phases of one job, `detsys.cross_phase_id` for
the phases of one Action, and the trace itself for the programs a phase runs.

The first Action to run makes the invocation ID and exports it to the job as
`$DETSYS_INVOCATION_ID`.
Every later step finds it there, and so does every program the steps run.