…`$THREAD_SUBPROCS=False` (#6112)

### Motivation

1. `@$()` is using captured_object that is working in parallel thread
and sometimes can produce race condition. We need to replace it back to
captured_stdout. cc #3512
2. Because of `$()` is working in `$THREAD_SUBPROCS=False` so `@$()`
should work.

Closes #4056
cc #4094

### Before

```xsh
$THREAD_SUBPROCS=False
showcmd $(echo '1\n2 3')  # working
# ['1\n2 3\n']
showcmd @$(echo '1\n2 3')  # not working
# 1    # not captured
# 2 3  # not captured
# <showcmd help>
```

### After

```xsh
$THREAD_SUBPROCS=False
showcmd $(echo '1\n2 3')  # working
# ['1\n2 3\n']
showcmd @$(echo '1\n2 3')  # working
# ['1', '2', '3']
```


## For community
⬇️ **Please click the 👍 reaction instead of leaving a `+1` or 👍
comment**

---------

Co-authored-by: Andy Kipp <123@321.123>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>