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/lib.rs

@@ -34,8 +34,6 @@ pub struct SpawnInTerminal {
     pub use_new_terminal: bool,
     /// Whether to allow multiple instances of the same task to be run, or rather wait for the existing ones to finish.
     pub allow_concurrent_runs: bool,
-    /// Whether the command should be spawned in a separate shell instance.
-    pub separate_shell: bool,
 }
 
 /// Represents a short lived recipe of a task, whose main purpose