Add explicit pkg-latest variant to build config #39

Job	Run time
build / detect	6s
build / build (pkg, Containerfile.pkg, amd64, latest, false, app, rsync)	11m 13s
build / build (pkg-latest, Containerfile.pkg, amd64, 15-latest, false, app, rsync)	9m 0s
build / test-compose	-1s
build / commit-sbom	4s
build / create-manifests	-1s
build / notify-status	4s
	20m 25s

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