Implement a CI test against Envoy's head commit

[mum4k]

Nighthawk's CI currently pins a specific commit in Envoy. If we add a separate CI workflow that will run against head of Envoy, we will get an early notification when Nighthawk no longer builds after Envoy changes.

Note - the intention here is to also keep performing Envoy update PRs regularly (weekly) as we do today. This new CI workflow can provide an additional signal when the rate of change in Envoy is high.