If the host only has RSA, use its RSA + generate ecdsa + ed25519, etc.
Perhaps fixes https://twitter.com/colek42c/status/1550554439299244032 and
something else that was reported.
Change-Id: I88dc475c8e3d95b6f25288ff7664b8e72655fd16
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>