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

[johnstcn]

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 which is marked for use by 'legacy API consumers'.

[johnstcn]

One possibility is a custom logging driver https://docs.docker.com/engine/extend/plugins_logging/