Fixed new FTB installer args and manifest

[itzg]

Fix regression from #3174

Applies info from FTBTeam/FTB-Modpack-Issues#6312 (comment)

[framerunner]

Currently closes the server/container, this is what it says in the logs:

[init] Running as uid=1000 gid=1000 with /data as 'drwxr-x--- 18 1000 1000 4096 Nov 15 19:48 /data'
[init] FTB modpack ID 126, version ID 100001 is ready to go
jq: error: Could not open file .manifest.json: No such file or directory

[itzg]

Ah, I'm sorry, I broke the legacy installer support with that and I'll see if I can fix that. For now you can set FTB_FORCE_REINSTALL to "true".

[itzg]

@framerunner this build should fix the issue https://github.com/itzg/docker-minecraft-server/actions/runs/12180940206