vm/gvisor: enable watchdog panic

We need it to crash to detect hangs.
diff --git a/vm/gvisor/gvisor.go b/vm/gvisor/gvisor.go
index 76e7a22..f077443 100644
--- a/vm/gvisor/gvisor.go
+++ b/vm/gvisor/gvisor.go
@@ -183,6 +183,7 @@
 func (inst *instance) runscCmd(add ...string) *exec.Cmd {
 	args := []string{
 		"-root", inst.rootDir,
+		"-watchdog-action=panic",
 		"-network=none",
 	}
 	if inst.cfg.RunscArgs != "" {