keep reruns on the same goroutine
keep reruns on the same goroutine

No need for this extra parallelism, and it's extra contention.

Cq-Include-Trybots: luci.skia.skia.primary:FM-Debian10-Clang-GCE-CPU-AVX2-x86_64-Debug-All,FM-Win2019-Clang-GCE-CPU-AVX2-x86_64-Debug-All
Change-Id: I5c0d52def5043555f313e99713335aa66b269e22
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/364014
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia