Group headers are not correctly aligned when using anchor_align: :end in Primer::Alpha::ActionMenu

[HDinger]

Steps to reproduce

• Create a Primer::Alpha::ActionMenu with anchor_align: :end and groups
• e.g:

<div style="display: grid; place-content: center;">
<%= render(Primer::Alpha::ActionMenu.new(anchor_align: :end)) do |component| %>
  <% component.with_show_button { "Menu" } %>
    <% component.with_group do |group|
      group.with_heading(title: "Vegetarian options")
      group.with_item(label: "Grilled portbello mushroom")
      group.with_item(label: "Polenta")
    end %>
<% end %>
</div>

Actual behavior

• The menu opens correctly positioned
• The items are correclty aligned on the left side
• The group header is aligned at the right side

Expected behaviour

• The group header should also be left aligned, no matter how the menu is positioned.