Fix systemd service installation hanging

* Fix systemd service installation hanging

---
This kills the hanging when there is a network issue (port blocking or no Internet) and the installation cannot be completed with no error sent to the output.

Before (killed manually since it hangs forever):
![499987567-de9003f9-4aaa-4667-9495-1d4b01069bed](/uploads/01063e6c2cf81fdd91ac8fbcd7f04a1b/499987567-de9003f9-4aaa-4667-9495-1d4b01069bed.png){width=817 height=69}

After:
![499986549-f031035f-1633-46c0-a896-d9fd37054e83](/uploads/00c273f37d415617104b44736921b3d7/499986549-f031035f-1633-46c0-a896-d9fd37054e83.png){width=825 height=78}
---

Author: cicku

cmd/cloudflared/linux_service.go

@@ -60,7 +60,7 @@ After=network-online.target
 Wants=network-online.target
 
 [Service]
-TimeoutStartSec=0
+TimeoutStartSec=15
 Type=notify
 ExecStart={{ .Path }} --no-autoupdate{{ range .ExtraArgs }} {{ . }}{{ end }}
 Restart=on-failure