Relative path does not work in AppImage version

[ztianyang]

SuperTux version: supertux2 v0.6.3-203-g1ac05224d
System information: Ubuntu 20.04.3

Expected behavior

Specifying a level using relative path in the command line should work.

Actual behavior

Specifying a level using relative path in the command line doesn't work.

Steps to reproduce actual behavior

1. Download a nightly AppImage build of SuperTux.
2. Make the AppImage file executable.
3. Place a level file in the same directory as the AppImage file.
4. Run SuperTux with that level file using the command line.

tianyang@ubuntu-laptop:~/Downloads$ ls
level1.stl  SuperTux-v0.6.3-203-g1ac05224d.glibc2.29-x86_64.AppImage
tianyang@ubuntu-laptop:~/Downloads$ ./SuperTux-v0.6.3-203-g1ac05224d.glibc2.29-x86_64.AppImage level1.stl 
[FATAL] /home/runner/work/supertux/supertux/src/supertux/game_session.cpp:151 Couldn't start level: Problem when reading level 'level1.stl': Couldn't open file 'level1.stl': not found
[FATAL] /home/runner/work/supertux/supertux/src/supertux/main.cpp:667 Unexpected exception: Initializing the level failed.
tianyang@ubuntu-laptop:~/Downloads$ 

Absolute path works, but relative path doesn't.

[Semphriss]

Path management in CLI is all over the place. Using --edit-level or not changes the way the game interprets the path (see #1819 ).

For the moment, it's better to use any method that "works" - but this should indeed be fixed.

[ztianyang]

Just to clarify, this issue only happens with the AppImage build of SuperTux, not with SuperTux compiled from source.