feat(task.nu): Trim | from the closure (#1184)

Also trim pipe characters (`|`) so it doesn't error out if the closure
is defined with empty args.
```nu
let $task = {||
	...
}
```

Author: Tyarel8

modules/background_task/task.nu

@@ -52,6 +52,7 @@ export def spawn [
   (
     view source $command
     | str trim --left --char "{"
+    | str trim --left --char "|"
     | str trim --right --char "}"
   )
   | save --force $source_path