network: fix nmcli check pattern (#472)

Given the implementation of cliExists() we are ignoring one case
by not checking the boolean return, if should assume nmcli doesn't
exist if the boolean return is false and not the error as nil.

Author: dorileo

google_guest_agent/network/manager/network_manager_linux.go

@@ -143,8 +143,8 @@ func (n *networkManager) IsManaging(ctx context.Context, iface string) (bool, er
 
 	// Check for existence of nmcli. Without nmcli, the agent cannot tell NetworkManager
 	// to reload the configs for its connections.
-	_, err := cliExists("nmcli")
-	if err != nil {
+	exists, err := cliExists("nmcli")
+	if !exists {
 		return false, err
 	}