kitchen: fix recipe bootstrap step start time
kitchen: fix recipe bootstrap step start time

TBR=martiniss@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2074893002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go