[led] Use new envvar to set debug sleep.
[led] Use new envvar to set debug sleep.

Since the bundle scripts switched to `exec`, the old hack no longer
worked. Recipe engine now supports an envvar which activates a sleep
quite high in its stack, so switch to that.

R=yiwzhang

Bug: 1197300
Change-Id: Ib97661efb06635f12218fe6eaff584d01c166cb8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3002696
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

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