Fix build sidebar build details padding

## What does this MR do?

Removes a `.block-first` overriding declaration that was added to fix the coverage padding and moved the padding that fixes the coverage block to a `.block.coverage` declaration.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

The build sidebar build details had too much padding as seen in #22529.

## Screenshots (if relevant)

![Screen_Shot_2016-09-24_at_19.38.40](/uploads/2c11a71f3022909fe8a5d1e983445667/Screen_Shot_2016-09-24_at_19.38.40.png)

![Screen_Shot_2016-09-24_at_19.39.06](/uploads/482af29f4bb7e284469466da59901087/Screen_Shot_2016-09-24_at_19.39.06.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #22529

See merge request !6506
Signed-off-by: Rémy Coutable <remy@rymai.me>