That way one can use environment variables in task definitions.

Fixes: #8660

/cc @SomeoneToIgnore it looks like we don't ever set `separate_shell` to
false anymore, it might be worth streamlining?

Release Notes:

- Fixed static tasks not being run under a separate shell.
- Removed `separate_shell` setting from task definitions. It is now a default for tasks defined in tasks.json file.