cmd/bench: generate branch tag

This change modifies cmd/bench to pick up BENCH_BRANCH from the builder
environment and prints the "branch" benchmark tag.

Change-Id: I3ca12ac91d98b6fd20e9ae06816e1cfe30364eb6
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/385734
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
1 file changed