Go 1.20: fix up retry certificate retry after Go CL "crypto/tls: add CertificateVerificationError to tls handshake"

[aktau]

Certificate retry inhibiting would be broken in Go 1.20.

The upstream CL http://go.dev/cl/449336 changed the error type returned when there's a certificate verification failure. The following line no longer hits:

go-retryablehttp/client.go

Lines 476 to 478 in 493aa4c

	if _, ok := v.Err.(x509.UnknownAuthorityError); ok {
	return false, v
	}

I believe it would resume working (and also work for older Go versions) if this was changed to:

			// Don't retry if the error was due to TLS cert verification failure.
-			if _, ok := v.Err.(x509.UnknownAuthorityError); ok {
+			if _, ok := errors.As(v.Err, &x509.UnknownAuthorityError{}); ok {
				return false, nil
			}