Fix `Security::FindingsFinder` for multiple report artifacts

Previosly we were using only the first security report artifact of a
build while finding the security findings, if there were more than one.
With this change, we are now using all the security report artifacts
of the same build while finding the security findings.

Changelog: fixed
EE: true

ee/app/finders/security/findings_finder.rb

@@ -96,10 +96,10 @@ module Security
     def report_findings
       @report_findings ||= begin
         builds.each_with_object({}) do |build, memo|
-          report = build.job_artifacts.map(&:security_report).compact.first
-          next unless report
+          reports = build.job_artifacts.map(&:security_report).compact
+          next unless reports.present?
 
-          memo[build.id] = report.findings.group_by(&:uuid).transform_values(&:first)
+          memo[build.id] = reports.flat_map(&:findings).index_by(&:uuid)
         end
       end
     end