feat(flatpak): High Tide cover image access (#55)

Adds a permission to access cover images from [High Tide](https://flathub.org/en-GB/apps/io.github.nokse22.high-tide)

Reviewed-on: https://codeberg.org/GeopJr/Turntable/pulls/55
Co-authored-by: Nokse <nokse@posteo.com>
Co-committed-by: Nokse <nokse@posteo.com>

Author: Nokse22

build-aux/dev.geopjr.Turntable.json

@@ -29,6 +29,7 @@
 		"--filesystem=~/.var/app/ca.edestcroix.Recordbox/data/Recordbox/:ro",
 		"--filesystem=~/.var/app/org.gnome.Rhythmbox3/cache/rhythmbox/album-art/:ro",
 		"--filesystem=~/.var/app/org.gnome.Lollypop/cache/lollypop/:ro",
+		"--filesystem=~/.var/app/io.github.nokse22.high-tide/cache/images:ro",
 		"--filesystem=~/.cache/:ro",
 		"--filesystem=xdg-data/applications",
 		"--filesystem=~/.local/share/xdg-desktop-portal:ro",