derp: set NotBefore and NotAfter in DERP server's metacert

Fixes regression from e415991256 that
only affected Windows users because Go only on Windows delegates x509
cert validation to the OS and Windows as unhappy with our "metacert"
lacking NotBefore and NotAfter.

Fixes #705

derp/derp_server.go

@@ -306,6 +306,9 @@ func (s *Server) initMetacert() {
 		Subject: pkix.Name{
 			CommonName: fmt.Sprintf("derpkey%x", s.publicKey[:]),
 		},
+		// Windows requires NotAfter and NotBefore set:
+		NotAfter:  time.Now().Add(30 * 24 * time.Hour),
+		NotBefore: time.Now().Add(-30 * 24 * time.Hour),
 	}
 	cert, err := x509.CreateCertificate(crand.Reader, tmpl, tmpl, pub, priv)
 	if err != nil {