cmd/bench: print pkg and shortname tags for Sweet benchmark results

Without these, perf.golang.org gets confused. The tags from bent runs
get applied to Sweet benchmarks.

Change-Id: I975526bfd9feb7015f1f88206b7cacfbc487addf
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/384974
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>