enhancement: replace agentsdk.LogSender direct integration with external log sender

Currently, envbuilder consumes `CODER_AGENT_URL`, `CODER_AGENT_TOKEN`, and `CODER_AGENT_SUBSYSTEM` environment variables to enable sending workspace build logs to Coder.

For Kubernetes, this can instead be handled by `coder/coder-logstream-kube`.
Investigate an alternative approach for envbuilder usage outside of Kubernetes (e.g. Docker).

This is also kind important because envbuilder currently references on [`agentsdk.ExternalLogSourceID`](https://github.com/coder/coder/blob/main/codersdk/agentsdk/agentsdk.go#L33) which is marked for use by 'legacy API consumers'.