fix: propagate include vars in multi-level includes

[hudclark]

Previously, included vars would be set to nil upon being included by another taskfile. They are now propagated up during the include process.

This is especially problematic for users who want to use library-like taskfiles in a monorepo.

├── common
│   └── docker.yml
├── service-a
│   └── Taskfile.yml 
├── service-b
│   └── Taskfile.yml
└── Taskfile.yaml

You can imagine that the contents of service-a/Taskfile.yml resemble

version: '3'
includes:
  docker:
    taskfile: ../common/docker.yml
    vars:
      DOCKER_IMAGE: service-a

Fixes:

• Vars of included Taskfiles in included Taskfiles  #778
• Vars don't get properly propagated with two levels of included nesting #996

[andreynering]

Nice work @hudclark, thanks a lot!