Add org.freedesktop.Flatpak permission

[bbhtt]

Fixes https://github.com/flathub/io.freetubeapp.FreeTube/issues/74

This isn't very ideal as it allows to run arbitrary commands from inside the sandbox. But there's no other solution I can think of since Freetube doesn't download the video, it only passes the link to the external player.

Tested with:

1. Flatpaked MPV https://github.com/FreeTubeApp/FreeTube/issues/1735#issuecomment-1141670904
2. Host MPV https://github.com/FreeTubeApp/FreeTube/issues/1735#issuecomment-1029445701

[flathubbot]

Started test build 103701

[bbhtt]

bot, build

[flathubbot]

Queued test build for io.freetubeapp.FreeTube.

[flathubbot]

Started test build 103703

[flathubbot]

Build 103703 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/101406/io.freetubeapp.FreeTube.flatpakref

[flathubbot]

Build 103701 failed

[flathubbot]

Started test build 4648

[flathubbot]

Build 4648 failed

[bbhtt]

Build 4648 failed

This is failing due to the linter, an exception must be requested.

[bbhtt]

cc @PrestonN

[bbhtt]

I'm closing this flatpak spawn is a huge access and doesn't make sense to me when a) Freetube can play the videos by itself B) External player is optional and there are too many to support C) Maybe it'd be better to include at least MPV in this flatpak

The alternative is to wait for a portal solution