Create dynamic fixture for build_spec

## What does this MR do?

Replace `spec/javascripts/fixtures/build.html.haml` by a dynamically created fixture (using `rake teaspoon:fixtures`).

## Why was this MR needed?

The existing fixture was not representing the real page. 

## What are the relevant issue numbers?

#24614 would have been avoided

following !6059

See merge request !7589