feat(default-flatpaks): Integrate flatpaks into ISO once it's supported by isogenerator

[fiftydinar]

Users would not need to wait for flatpaks to install once they boot into the system.
That installation step will be in the ISO installer screen instead.

Post-install systemd service would still remain, as it's still needed for applying updated changes from default-flatpaks recipe.
It's also needed to support image rebase installation method.

Related issue:
ublue-os/isogenerator#9

[xynydev]

This should be actually part of the ISO generation action if these Flatpaks are configured at ISO generation time and not in the image.

[fiftydinar]

This should be actually part of the ISO generation action if these Flatpaks are configured at ISO generation time and not in the image.

You're right, I transfered this issue to this one:
blue-build/github-action#20

[xynydev]

Did you know that you can actually transfer issues, including the discussion, using a button down below? Anyways, nothing was lost here, good call on moving the issue!

[fiftydinar]

Did you know that you can actually transfer issues, including the discussion, using a button down below? Anyways, nothing was lost here, good call on moving the issue!

I thought that only those with permissions could do so, like moderators or maintainers.
I learnt something new at least.

[xynydev]

Oh yeah, this repo isn't moved yet so can't...