Fix bug in subprocess runner.
Fix bug in subprocess runner.

StepConfig is no longer a namedtuple, and so doesn't have a ._replace
method.

Introduced in https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1542930

R=jchinlee@chromium.org, f3fbe4c4218d2421400bf0bd07366865fd7a0c7e@chromium.org

Bug: 910369
Change-Id: I7cb03d6bda61001585e9d35a623916c8be399e45
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1582521
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Stephen Martinis <f3fbe4c4218d2421400bf0bd07366865fd7a0c7e@chromium.org>
Reviewed-by: Stephen Martinis <f3fbe4c4218d2421400bf0bd07366865fd7a0c7e@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/recipes-py,luci/recipes-py