connect: NOMAD_envoy_version only set if task driver is docker

[shoenig]

In the envoy_bootstrap_hook we have logic that prevents making the query to Consul for its desired envoy version if the task driver of the sidecar_task is not explicitly "docker". Perhaps a better heuristic would be to check if task.config["image"] exists, or maybe just always set the environment variable regardless of task driver. The downside to always setting the version environment variable is that users of non-image based task drivers (e.g. exec) will cause queries to Consul for no reason.

[shoenig]

Closed by #17041

[github-actions]

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