use correct certificate extension when getting cluster of agentless node

[capnspacehook]

lib/utils.CertTeleportClusterName is set by the SSH user key auth handlers, so it should always be set.

Updates #24778.

[nklaassen]

Tried this out and it works 👍 Do you think there's an appropriate test we could add to somehow check that the certs generated by tctl auth sign --format openssh can be used successfully? I'm alright with merging this fix in without it, so we can use it for the rest of the test plan, and adding a test later

[capnspacehook]

I'm pretty sure the certificate used here is the user's Teleport issued SSH certificate, not the OpenSSH server's copy of the OpenSHH CA public key. I tried to create a regression test mimicking how tsh proxy ssh would connect to an agentless node, but it didn't fail before the change. So not sure how to test it tbh. I think I'll merge this for now and we can add a test later potentially as you said.

[strideynet]

Does this need backporting ??

[strideynet]

This PR does not need backporting as it is superceded by #24935