ssh/agent: fix non-RSA certificates

The type of ssh.PublicKey.Type can be a certificate type, while the
algorithm passed to SignWithAlgorithm is going to be an underlying
algorithm.

Fixes golang/go#52185

Change-Id: I0f7c46defa83d1fd64a3c1e861734650b20cca21
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/404614
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>

ssh/certs.go

@@ -460,6 +460,8 @@ func (c *Certificate) SignCert(rand io.Reader, authority Signer) error {
 
 // certKeyAlgoNames is a mapping from known certificate algorithm names to the
 // corresponding public key signature algorithm.
+//
+// This map must be kept in sync with the one in agent/client.go.
 var certKeyAlgoNames = map[string]string{
 	CertAlgoRSAv01:        KeyAlgoRSA,
 	CertAlgoRSASHA256v01:  KeyAlgoRSASHA256,