Update screenshot location to builddir/files/share/app-info/media

[bbhtt]

See https://github.com/flatpak/flatpak-github-actions/issues/168

The location was changed in the libappstream port of flatpak-builder

https://github.com/flatpak/flatpak-builder/commit/411da7541456b7acea64d5edf1e68386b2eeb0ea

[TingPing]

The contents of the two directories are different btw.

> tree _build/files/share/app-info/media/
└── com
    └── viber
        └── Viber.desktop
            └── 57afa5ae90ff56ff22f259783cfd3c37
                ├── icons
                │   └── 128x128
                │       └── com.viber.Viber.desktop.png
                └── screenshots
                    ├── image-1_224x214.png
                    └── image-1_orig.png

> tree _build2/screenshots/
└── com.viber.Viber-master
    ├── 112x63
    │   └── com.viber.Viber-29b4518dc53f788e03cacc3f0bb323bc.png
    ├── 1248x702
    ├── 1504x846
    ├── 224x126
    │   └── com.viber.Viber-29b4518dc53f788e03cacc3f0bb323bc.png
    ├── 624x351
    │   └── com.viber.Viber-29b4518dc53f788e03cacc3f0bb323bc.png
    └── 752x423
        └── com.viber.Viber-29b4518dc53f788e03cacc3f0bb323bc.png

[bbhtt]

It should work fine, appstream now generates the urls in catalogue as /media/com/viber/<hash>/screenshot/<image>