chore: replace x86_64-only Flatpaks with amd64 alternatives

[p5]

I'm not too sure how the overrides work, so please give this a good review :)

This replaces Firefox with Chromium (the next best option for a browser).
And removes Thunderbird, since it's not really needed when you have a browser.

[p5]

Copied from the ublue-os/packages file, with Firefox replaced with Chromium.

[tulilirockz]

I think we can add this to the package itself when using aarch64. Putting a pin on this

[tulilirockz]

Nono!

[tulilirockz]

Wanted to talk to p5 about this but I think this PR looks good

[tulilirockz]

It looks good! Im just thinking like, the stuff on overrides should be build on top of the existing build, everything that is not explicitly a override should work, always, no matter arch, no matter the overrides. putting the file copying before makes the non-overrides build "impure", thus it can break when certain files are or are not present on the overrides, that locks us into having those always when building images with the overrides like -hwe.

For example, imagine we have some systemd configuration that needs to be there specifically for -dx because of some program we added 10 months ago, and we dont have any idea why not having it breaks the previous steps when building -dx (e.g.: image-info.sh or something), that would lock us into having that file forever, instead of doing an "whitelist" pattern with the overrides

[tulilirockz]

I believe we can just have a sed -i statement on a script under build_script/aarch64 instead of having this file here - think about the defaults, it should always be the non-overrides, then maybe do stuff like this if strictly necessary. In this case this would just add a bit of overhead when maintaining flatpaks because we would need to manually compare the files and see what we want.

[tulilirockz]

Was this necessary? Usually rsync output is more verbose, I get not relying on it but I think we might need this here?