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/taskfile.go

@@ -7,8 +7,8 @@ type Taskfile struct {
 	Output     string
 	Method     string
 	Includes   IncludedTaskfiles
-	Vars       Vars
-	Env        Vars
+	Vars       *Vars
+	Env        *Vars
 	Tasks      Tasks
 	Silent     bool
 }
@@ -21,8 +21,8 @@ func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		Output     string
 		Method     string
 		Includes   IncludedTaskfiles
-		Vars       Vars
-		Env        Vars
+		Vars       *Vars
+		Env        *Vars
 		Tasks      Tasks
 		Silent     bool
 	}
@@ -41,8 +41,5 @@ func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if tf.Expansions <= 0 {
 		tf.Expansions = 2
 	}
-	if tf.Vars == nil {
-		tf.Vars = make(Vars)
-	}
 	return nil
 }