cmd/bench: emit builder name as benchmark key

The purpose of emitting the builder name is to help group results
together further down the pipeline, specifically to produce a useful
geomean.

Change-Id: I99c04012ba48e795768b824d073f84af303dcfc3
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/627516
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
1 file changed