Include .flatpak in artifact name to differentiate from other artifacts #10

Job	Run time
Flatpak Builder Stop At	1m 37s
Flatpak Builder (x86_64, use-cache, cache-restored)	1m 36s
Lint	19s
Flatpak Builder (x86_64, use-cache, no-cache-restored)	1m 42s
Flatpak Builder (x86_64, no-cache, no-cache-restored)	1m 33s
Tests (16.x)	14s
Flatpak Builder (aarch64, use-cache, cache-restored)	2m 38s
Tests (18.x)	17s
Tests (20.x)	16s
Flatpak Builder Cache Hit	0s
	10m 12s

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