Merge branch 'add-ubuntu-wallpapers' into 'master'

Add the Ubuntu wallpapers recipe

See merge request !365

recipes/artwork/ubuntu-wallpapers/recipe.toml

+[source]
+tar = "https://launchpad.net/ubuntu/+archive/primary/+sourcefiles/ubuntu-wallpapers/23.10.4/ubuntu-wallpapers_23.10.4.orig.tar.gz"
+blake3 = "1e479d0aa48fe3f2961a2dac28c3ed397a29616cf6e7d73f5ceb6fabfd6449e1"
+[build]
+template = "custom"
+script = """
+mkdir -pv "${COOKBOOK_STAGE}"/usr/share/wallpapers
+cp -rv "${COOKBOOK_SOURCE}"/ubuntu-wallpapers-23.10.4/*.{jpg,png} "${COOKBOOK_STAGE}"/usr/share/wallpapers
+"""