[swarming] fix tasklist.exe being called on non-Windows bots
[swarming] fix tasklist.exe being called on non-Windows bots

https://console.cloud.google.com/errors/CJOyh-_s3onKmAE?project=chromium-swarm&time=P30D&utm_source=cloud-notification&utm_medium=email&utm_content=new-error

Change-Id: I26a89d92154299d3b1517856e7facfbe50b963fa
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2893927
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py