Allow setting STACK_WORK as a global env for --work-dir

[domenkozar]

Dealing with MAX_PATH issue on windows mean setting short STACK_ROOT, but working directory also can trigger the limit.

See #1947

Currently I pass --work-dir to all stack commands, but it would be more convenient to also set STACK_WORK.

[mgsloan]

Sure, I've added support for a STACK_WORK env var.

[domenkozar]

Thanks!