siso: fix StepMetric.RunTime to include postproc for remoteexec

For local exec and remote cache, RunTime already included postproc.

Hence for remote exec to not include it was inconsistent, and also goes
against the intention of RunTime being "the total duration of the action
execution, including overhead such as uploading / downloading files".

A previous CL introduced a `-run` span for remoteexec, but it would have
to overlap the last attempt + postproc.
Rather than trying to contort the last attempt to be parented to the
`-run` span, let all the `-attempt` spans be siblings, have another
`-postproc` sibling span.

Then, the responsibility for deciding `-run` is pushed to where RunTime
is projected.

Bug: b/520207778
Change-Id: Ia3f07f1ceef3733c37cd8341752a99e16a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8160704
Commit-Queue: Richard Wang <richardwa@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>