Make dynamic variables run on the right directory

It was always running in the main Taskfile dir, even when the variable was
declared in an included taskfile in another directory or when task had a
custom dir.

Closes #384

internal/compiler/compiler.go

@@ -8,6 +8,6 @@ import (
 // E.g. variable merger, template processing, etc.
 type Compiler interface {
 	GetVariables(t *taskfile.Task, call taskfile.Call) (*taskfile.Vars, error)
-	HandleDynamicVar(v taskfile.Var) (string, error)
+	HandleDynamicVar(v taskfile.Var, dir string) (string, error)
 	ResetCache()
 }