Salvo: Build Envoy and Nighthawk from local disk path.

[gyohuangxin]

Currently, we provides the way to build Envoy and Nighthawk binary from remote location (one commit in a repo) : https://github.com/envoyproxy/envoy-perf/tree/main/salvo#binary-benchmark.

It's also useful to support quick testing of code changes in the local disk paths without pushing them as commits. It seems that Salvo already has the API to build from local disk path: https://github.com/envoyproxy/envoy-perf/blob/main/salvo/api/source.proto#L29, we should verify it works and provide one example in Salvo's doc.