Provide way to substitute ISOLATED_OUTDIR with "swarming.py reproduce"

[kenrussell]

The Chromium GPU testing docs are being updated:
https://www.chromium.org/developers/testing/gpu-testing

The easiest way to explain to end users how to run a job locally that previously ran on the swarming bots is via "swarming.py reproduce". However, this doesn't work for GPU jobs like this one:
https://chromium-swarm.appspot.com/user/task/2d23cb9f3d7ca610

because the command line arguments include:
--isolated-script-test-output=${ISOLATED_OUTDIR}/output.json

and there's no way to provide the output directory to "swarming.py reproduce".

This is urgent. Would appreciate a quick fix. Thanks.