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/oneshot_source.rs

@@ -47,7 +47,6 @@ impl Task for OneshotTask {
             env: Default::default(),
             use_new_terminal: Default::default(),
             allow_concurrent_runs: Default::default(),
-            separate_shell: true,
         })
     }
 }