isolate: do not use more goroutines than cpu cores
isolate: do not use more goroutines than cpu cores

ComputeMerkleTree is cpu intensive function, so I suspect it is better
to avoid using too many goroutines there.

Bug: 1193375
Change-Id: I94994adb9c6d8dd17abb48491c6adbb8fd1851f3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2790298
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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