Allow overriding of global variable default at the task level

[luiavila]

Description

Example situation:

By default, the global variable default should be used. If a task defines / attempts to override the default, it is currently not utilized.

version: '3'

vars:
  MY_VAR: '{{.MY_VAR | default 123}}'

tasks:
  MY_TASK:
    vars:
      MY_VAR: '{{.MY_VAR | default 456}}'
    cmds:
      - echo "MY_VAR = {{.MY_VAR}}"

Current output:

> task MY_TASK
MY_VAR = 123

> task MY_TASK MY_VAR=789
MY_VAR = 789