Remove btn-build class

What does this MR do?

Fix for #300263 (closed) .

The .btn-build class is an old class that could be removed from the codebase to use new UI patterns.

Every .btn-build is a button that contains an icon:

before	after
%button: gl-button btn btn-default btn-build ml-0	%button: gl-button btn btn-default btn-icon + %svg: gl-icon

• btn-icon gives this the right width according to Pajamas for icon buttons
• btn-default-secondary replaces the previously forced $gl-text-color-secondary of the icon svg.

Additionally I add a .btn-group to the jobs page, to remove the space between the buttons and make the look more consistent with the rest of the UI.

Screenshots (strongly suggested)

Note: I suggest opening the images and checking them side by side, the main difference is a change in the width of the buttons.

	Before	After
Artifacts page
Job page (simple)
Job page (delayed jobs)
Job page (pending job)
Failed jobs (no difference here)
Environment Redeploy

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team