Support for specifying a custom Flatpak manifest to build and install

[orowith2os]

I would like to be able to specify a source link, manifest link, and potentially pre-build commands to run, and have nix-flatpak build and install the resulting Flatpak bundle.

I would make much use of this in testing mesa-git with custom branches or patches.

[gmodena]

Hey @orowith2os,

This could be interesting to support in the future, but there's a few things I'd like nix-flatpak to support before increasing scope (notably: #135 and #82#82 Do you have an example of manifest and build commands you are using with mesa-git?

I am not much familiar with the flatpak build systems, so I'd have to learn that a bit more to understand what this feature would imply.

[orowith2os]

I originally started this a while back with this, it might be useful: https://github.com/orowith2os/mesa-asahi-flatpak

[gmodena]

thanks for the pointer!