Refactor variables: Keep order of declaration

This shouldn't have any behavior changes for now. This is a code
refactor that should allow us to do further improvements on how
variables are handled, specially regarding respecting the declaration
order in Taskfiles, which should make it easier for the users.

Initial work on #218

internal/taskfile/task.go

@@ -19,8 +19,8 @@ type Task struct {
 	Status        []string
 	Preconditions []*Precondition
 	Dir           string
-	Vars          Vars
-	Env           Vars
+	Vars          *Vars
+	Env           *Vars
 	Silent        bool
 	Method        string
 	Prefix        string
@@ -55,8 +55,8 @@ func (t *Task) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		Status        []string
 		Preconditions []*Precondition
 		Dir           string
-		Vars          Vars
-		Env           Vars
+		Vars          *Vars
+		Env           *Vars
 		Silent        bool
 		Method        string
 		Prefix        string