fix(cli/server): do not redirect /healthz

[johnstcn]

If you set --redirect-access-url=true, then you would have seen these warnings from Kubelet:

   Warning  ProbeWarning  5m52s (x121 over 25m)  kubelet            Liveness probe warning: Probe terminated redirects, Response body: <a href="https://dev.coder.com">Temporary Redirect</a>.                                             │
│   Warning  ProbeWarning  52s (x152 over 26m)    kubelet            Readiness probe warning: Probe terminated redirects, Response body: <a href="https://dev.coder.com">Temporary Redirect</a>.   

We have specific checks for /derp to not redirect in this case, so extending this for /healthz.