Incorrect timestamps in provenance when running --provenance together with --cachedir

Hello,

When generating a research object of a workflow run which uses cached output from previous runs, the resulting timestamps are incorrect and correspond only to the final run which produced the RO.
This is undesirable, e.g. given that the metadata now gives incorrect duration of step execution.

## Expected Behavior
- primary.cwlprov.* contains timestamps of initial execution (when the cached results were generated)

## Actual Behavior
- primary.cwlprov.* contains timestamps of final execution (workflow run which generated the research object)

## Workflow Code
See [this example workflow + RO](https://github.com/RenskeW/cwlprov-provenance/tree/f5dd87a950eeaf7f96bd39dc218164832ff3cbea/cachedir_timestamps) for the details

## Your Environment
* cwltool version: cwltool 3.1.20220502060230