feat: add TASK_OFFLINE env and expose it as a special variable

[vmaerten]

Two things in this PR (very correlated) :

• Add a TASK_OFFLINE env variable to control --offline flag. Is it useful when you want to use --offline all the time. And also, It will be picked up by the autocomplete without modifying the autocomplete file (related to #1713) The CLI flags takes precedences over the env variable, this way we can override it as needed
• Added a special variable CLI_OFFLINE to expose this flag to the templating system (closes #1470)