Linux script: executable_name validation check fails when cwd is different and executable_name is relative.

[Sodium-Aluminate]

BepInEx/BepInEx#1189

when the script check the executable_name, it's using cwd from the parent process and may failed if not start this script from the dir of the script self.

we need check the $(abs_path $"executable_name") exists.

[arrowmaster]

executable_name is not intended to be a path but the name of the executable in the same directory as the script