cmd/bench: add repository key
cmd/bench: add repository key

The repository key specifies which Go repository (go, tools, sys, etc)
is getting benchmarked. Right now we only benchmark go, but this is
added in anticipation of future support for other repositories.

For golang/go#53538

Change-Id: I795323a65b776b001742d330cae2fc7f0c6d910c
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/413915
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/benchmarks,golang/benchmarks