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
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
