Flatpak support prototype

[valpackett]

Hi,

you might've seen me in the nixpkgs PR thread testing things, but after encountering some way too cursed issues (Python scripts being run with Bash?!) I started working on Flatpak instead.

So now I have a janky prototype setup that uses a FLATPAK_BWRAP wrapper to inject a FEX build into an x86_64 environment…

Here: https://codeberg.org/valpackett/fexwrap

And it even supports thunks! Sort of. There are actually some cursed issues with thunks to work on there. But this already provides a good baseline environment for further research into non-RootFS environments \o/