Allow builds right sidebar collapsing

Issue #3981 (closed) made it possible to collapse/expand right sidebar on some pages.

It would be cool to be allowed to do so on build pages. Indeed as the following screenshot shows, a lot of space is wasted (in the lower right corner, but this appears even more acute when scrolling down, the sidebar is just a white wasted area) by the right sidebar (at least in the case of vertical screens):