blueprint: remove omitempty from Customizations

See BurntSushi/toml#360

A recent change in BurntSushi/toml made encoding fail (later changed to
error) if a struct is marked as omitempty and is comparable. Go docs about
equality: https://go.dev/doc/go1#equality. Basically: A struct is comparable
if all of its fields are comparable. Slices are not comparable.

Customizations are marked as omitempty but they contain a lot of slices,
thus they are not comparable. The new version of BurntSushi/toml therefore
panics when we encode them.

The solution is to remove the omitempty tag from Customizations.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

internal/blueprint/blueprint.go

@@ -19,7 +19,7 @@ type Blueprint struct {
 	Modules        []Package       `json:"modules" toml:"modules"`
 	Groups         []Group         `json:"groups" toml:"groups"`
 	Containers     []Container     `json:"containers,omitempty" toml:"containers,omitempty"`
-	Customizations *Customizations `json:"customizations,omitempty" toml:"customizations,omitempty"`
+	Customizations *Customizations `json:"customizations,omitempty" toml:"customizations"`
 	Distro         string          `json:"distro" toml:"distro"`
 }