Fix Nomad server health check

[schmichael]

I think I caused a regression in #2467 -- from gitter:

git blame points to your commit 77d9b41 apparently before that change, servers did /v1/status/peers and clients /v1/agent/servers, but it got consolidated into /v1/agent/servers, which seems to be implemented in client only - so consul server health checks will fail on trunk
would it make sense to just switch the URI back to /v1/status/peers, as that seems to be a "generic" status check endpoint?

[shilov]

Can confirm this is causing "Nomad Client HTTP Check" to fail for nomad agents running in server mode.

As implied, changing /v1/agent/servers to /v1/status/peers in command/agent/agent.go fixes the issue.

[github-actions]

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