cmd/coordinator: add subrepo benchmarking support
cmd/coordinator: add subrepo benchmarking support

When running against a subrepo other than x/benchmarks, we check out an
experiment and baseline version of that subrepo for benchmarking. Note
that we are comparing two versions of the subrepo built with a single
version of Go (the latest release). We are not testing vs Go tip.

See comment in runBenchmarkTests for more details.

For golang/go#53538.

Change-Id: I4099cf99f763978e87658f3c9c3b5c362dc5042e
Reviewed-on: https://go-review.googlesource.com/c/build/+/442258
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Auto-Submit: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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