[BUG] Can't use `for` in a custom widget that delegates to `box`

[Nemo157]

Checklist before submitting an issue

• [X] I have searched through the existing closed and open issues for eww and made sure this is not a duplicate
• [X] I have specifically verified that this bug is not a common user error
• [X] I am providing as much relevant information as I am able to in this bug report (Minimal config to reproduce the issue for example, if applicable)

Description of the bug

When defining a custom widget that delegates its children into a box, using that widget with a for as child fails:

error: This widget can only be used as a child of some container widget such as box
  ┌─ /nix/store/xi81k4i5a1pa8l6hqajg1088lf0bflz1-hm_eww/eww.yuck:5:10
  │
5 │   (v-box (for workspace in workspaces (label :text {workspace})))
  │          ───────────────────────────────────────────────────────
  │
  → Hint: try wrapping this in a `box`

Reproducing the issue

(defwidget v-box [] (box :orientation "v" (children)))

(defvar workspaces '["foo"]')
(defwindow taskbar
  (v-box (for workspace in workspaces (label :text {workspace})))
)

Expected behaviour

A widget wrapping a box should be allowed to accept for as children.

Additional context

No response

[tokyob0t]

Same