Update screenshot location to builddir/files/share/app-info/media #572

Job	Run time
Flatpak Builder (x86_64, use-cache, cache-restored)	3m 54s
Flatpak Builder (x86_64, use-cache, no-cache-restored)	3m 53s
Lint	20s
Flatpak Builder (x86_64, no-cache, no-cache-restored)	1m 40s
Flatpak Builder (aarch64, use-cache, cache-restored)	6m 15s
Flatpak Builder Stop At	1m 37s
Tests (16.x)	12s
Tests (18.x)	19s
Tests (20.x)	15s
Flatpak Builder Cache Hit	1m 48s
	20m 13s

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