ScummVM Flatpak support

[lavadrop]

It seems flatpak's sandboxing is affecting Roberta.

Steam errors from terminal:

/bin/sh\0-c\0/home/user/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=63610 -- /home/user/.local/share/Steam/ubuntu12_32/steam-launch-wrapper -- '/home/user/.local/share/Steam/compatibilitytools.d/roberta'/run-vm --wait-before-run '/home/user/Games/SteamLibrary/steamapps/common/Riven/scummvm.exe' -c myst25.ini riven\0
chdir "/home/user/Games/SteamLibrary/steamapps/common/Riven"
ERROR: ld.so: object '/home/user/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
ERROR: ld.so: object '/home/user/.local/share/Steam/ubuntu12_64/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS64): ignored.
ERROR: ld.so: object '/home/user/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
ERROR: ld.so: object '/home/user/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
roberta: working dir: "/home/user/Games/SteamLibrary/steamapps/common/Riven"
roberta: original command: ['/home/user/Games/SteamLibrary/steamapps/common/Riven/scummvm.exe', '-c', 'myst25.ini', 'riven']
roberta: screen '0': 1920x1080
roberta: selected screen '0'
roberta: /home/user/Games/SteamLibrary/steamapps/common/Riven
roberta: scummvm.exe
roberta: ['/home/user/Games/SteamLibrary/steamapps/common/Riven/scummvm.exe', '-c', 'myst25.ini', 'riven']
roberta: working dir: "/home/user/Games/SteamLibrary/steamapps/common/Riven"
roberta: error: [Errno 2] No such file or directory: 'scummvm'