Allow --unstable to be passed through via environment variables

[p5]

It would be great if users can globally opt-in to unstable features by setting an environment variable such as JUST_ALLOW_UNSTABLE.

If that variable is set to any value, it acts the same as if the --unstable flag is passed through to the commands.
This still allows the safety of feature-flagging the newest commands, but those who want the absolute latest-and-greatest can set this option system-wide and not have to think about passing through the --unstable parameter

[Taywee]

This would also be useful for calling just inside of a recipe and being sure the unstable option propagates through.

[ealtintas]

I think using a directive like set unstable in a parent justfile (which may include other justfiles) would also a be good solution for this issue.

error: The !include directive is currently unstable. Invoke `just` with the `--unstable` flag to enable unstable features.

How long do you think the '!include' directive should stay as 'unstable' ? @casey

[ealtintas]

Related: #1430 , #1564

[casey]

Fixed in #1588!