task: Spawn static tasks in separate shell (#8827)

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.

crates/task/src/static_source.rs

@@ -34,7 +34,6 @@ impl Task for StaticTask {
             command: self.definition.command.clone(),
             args: self.definition.args.clone(),
             env: self.definition.env.clone(),
-            separate_shell: false,
         })
     }