Add missing timezone to legacy artifacts (ci_builds)

- ci_builds.artifacts_expire_at are copied to
  ci_job_artifacts.expire_at with incorrect timestamps when the
  database timezone is NOT utc

- ci_builds.artifacts_expire_at is `timestamp without time zone` and
  ci_job_artifacts.expire_at is `timestamp with time zone` on
  postgresql

- Tests fail locally for `rspec
  ./spec/lib/gitlab/import_export/import_export_spec.rb` without this
  change

lib/gitlab/background_migration/migrate_legacy_artifacts.rb

@@ -39,10 +39,10 @@ module Gitlab
           SELECT
               project_id,
               id,
-              artifacts_expire_at,
+              artifacts_expire_at #{add_missing_db_timezone},
               #{LEGACY_PATH_FILE_LOCATION},
-              created_at,
-              created_at,
+              created_at #{add_missing_db_timezone},
+              created_at #{add_missing_db_timezone},
               artifacts_file,
               artifacts_size,
               COALESCE(artifacts_file_store, #{FILE_LOCAL_STORE}),
@@ -81,10 +81,10 @@ module Gitlab
           SELECT
               project_id,
               id,
-              artifacts_expire_at,
+              artifacts_expire_at #{add_missing_db_timezone},
               #{LEGACY_PATH_FILE_LOCATION},
-              created_at,
-              created_at,
+              created_at #{add_missing_db_timezone},
+              created_at #{add_missing_db_timezone},
               artifacts_metadata,
               NULL,
               COALESCE(artifacts_metadata_store, #{FILE_LOCAL_STORE}),
@@ -121,6 +121,12 @@ module Gitlab
               AND artifacts_file <> ''
         SQL
       end
+
+      def add_missing_db_timezone
+        return '' unless Gitlab::Database.postgresql?
+
+        'at time zone \'UTC\''
+      end
     end
   end
 end