Fix failing solidus_admin thumbnail spec

Failure: /spec/components/solidus_admin/ui/thumbnail/component_spec.rb:6

Error:

/home/circleci/solidus/admin/spec/components/previews/solidus_admin/ui/thumbnail/component_preview/overview.html.erb:53:
numbered parameter is already used in
/home/circleci/solidus/admin/spec/components/previews/solidus_admin/ui/thumbnail/component_preview/overview.html.erb:52:
outer block here Failure/Error: <% image = Spree::Image.new.tap {
_1.define_singleton_method(:attachment) { attachment } } %>

By replaing the second instance of numbered params, we can avoid this
error with no behavioural or functional changes to the code.

Interestingly though, it was only failing on 2 out of our 8 spec runs,
and it did so consistently.

(cherry picked from commit 7176948)

admin/spec/components/previews/solidus_admin/ui/thumbnail/component_preview/overview.html.erb

@@ -50,7 +50,7 @@
 <div class="flex gap-4">
   <% product = Spree::Product.new(name: "A good product") %>
   <% attachment = Object.new.tap { def _1.url(*) "https://placekitten.com/280/200"; end }  %>
-  <% image = Spree::Image.new.tap { _1.define_singleton_method(:attachment) { attachment } } %>
+  <% image = Spree::Image.new.tap { |img| img.define_singleton_method(:attachment) { attachment } } %>
   <% [
     Spree::PromotionAction.new,
     Spree::UnitCancel.new,