feat: process variables in include vars

[pd93]

Fixes #2108

Including a Taskfile with a static variable works:

version: 3

includes:
  bar:
    taskfile: included.yaml
    vars:
      FOOBAR: foo

However, if you want to use an existing variable when including a Taskfile:

version: 3

vars:
  FOOBAR: foo

includes:
  bar:
    taskfile: included.yaml
    vars:
      FOOBAR: "{{.FOOBAR}}"

you're out of luck, since Task doesn't resolve variables in the include vars section. This PR adds that functionality.