dashboard: revert perf builder back to c2-standard-8
dashboard: revert perf builder back to c2-standard-8

CL 419077 changed the perf builder from c2-standard-8 to c2-standard-16.
This is not desirable for two reasons:

1. It changed the names of many of our benchmarks (e.g., ZapJSON-8 ->
   ZapJSON-16), breaking our dashboards.
2. More importantly, this change is almost certain to change performance
   characteristics which we are trying to keep consistent between runs.

Change-Id: If6a283ef02e0b7fed170e17d01930302d72e79b8
Reviewed-on: https://go-review.googlesource.com/c/build/+/420798
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>

golang/build,golang/build,golang/build,golang/build