ssh/tailssh: handle session recording when running in userspace mode

Previously it would dial out using the http.DefaultClient, however that doesn't work
when tailscaled is running in userspace mode (e.g. when testing).

Updates tailscale/corp#9967

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ssh/tailssh/tailssh_test.go

@@ -237,6 +237,10 @@ var (
 	testSignerOnce sync.Once
 )
 
+func (ts *localState) Dialer() *tsdial.Dialer {
+	return nil
+}
+
 func (ts *localState) GetSSH_HostKeys() ([]gossh.Signer, error) {
 	testSignerOnce.Do(func() {
 		_, priv, err := ed25519.GenerateKey(rand.Reader)