[led] Use LookPath to prefer python3 if available when bundling recipes.
[led] Use LookPath to prefer python3 if available when bundling recipes.

R=bryner, yiwzhang, yuanjunh

Bug: 1099143
Change-Id: I73d7d77731ce3a121832282522c51f1f879cb6ec
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3486425
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@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