Expose finished_at in job details JSON

This fixes a bug where the time a job has been running (including when
it is complete), the running time, is always labelled **Elapsed time**
in the job sidebar. This, however, is not intended following the
change of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75811,
and the running time should be labelled **Elapsed time** only when the
job is running and default to the **Duration** text.

The bug is fixed by adding `finished_at` to the `BuildDetailsEntity`
because it was missing there.

Changelog: fixed

app/serializers/build_details_entity.rb

 # frozen_string_literal: true
 
 class BuildDetailsEntity < Ci::JobEntity
-  expose :coverage, :erased_at, :duration
+  expose :coverage, :erased_at, :finished_at, :duration
   expose :tag_list, as: :tags
   expose :has_trace?, as: :has_trace
   expose :stage

spec/serializers/build_details_entity_spec.rb

@@ -29,7 +29,7 @@ RSpec.describe BuildDetailsEntity do
     end
 
     it 'contains the needed key value pairs' do
-      expect(subject).to include(:coverage, :erased_at, :duration)
+      expect(subject).to include(:coverage, :erased_at, :finished_at, :duration)
       expect(subject).to include(:runner, :pipeline)
       expect(subject).to include(:raw_path, :new_issue_path)
     end