Update linuxkit version:

linuxkit 1.6.0 is available for use.
This has at least one useful feature
related to ISO building that is needed.

Also, make the linuxkit version configurable.

Signed-off-by: Jacob Weinstock <[email protected]>

Author: jacobweinstock

build.sh

@@ -40,7 +40,7 @@ declare -g HOOK_LK_CONTAINERS_OCI_BASE="${HOOK_LK_CONTAINERS_OCI_BASE:-"quay.io/
 declare -g SKOPEO_IMAGE="${SKOPEO_IMAGE:-"quay.io/skopeo/stable:v1.17.0"}" # See https://quay.io/repository/skopeo/stable?tab=tags&tag=latest
 
 # See https://github.com/linuxkit/linuxkit/releases
-declare -g -r LINUXKIT_VERSION_DEFAULT="1.5.3" # LinuxKit version to use by default; each flavor can set its own too
+declare -g -r LINUXKIT_VERSION_DEFAULT="${LINUXKIT_VERSION:-"1.6.0"}" # LinuxKit version to use by default; each flavor can set its own too
 
 # Directory to use for storing downloaded artifacts: LinuxKit binary, shellcheck binary, etc.
 declare -g -r CACHE_DIR="${CACHE_DIR:-"cache"}"