Flag-file compatability

In already ported challenges, this was handled in a entrypoint-script.
The flag is being taken from the environment and passed into a file, e.g. https://github.com/CyberLeagueAustria/cyberleague-247-challenges/blob/main/pwn/p1/challenge/entrypoint.sh (note, this is also a challenge using nsjail)

Something similar should be in order to preserve compatibility.