Fix security report fetching in MRs

It seems another query in the MR page was added recently which included
an `id` field for the head pipeline, whereas the security reports query
did not include that field. This caused Apollo's cache write to fail,
since it couldn't reconcile the two fetched entities.

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/326071.

app/assets/javascripts/vue_shared/security_reports/queries/security_report_download_paths.query.graphql

@@ -6,6 +6,7 @@ query securityReportDownloadPaths(
   project(fullPath: $projectPath) {
     mergeRequest(iid: $iid) {
       headPipeline {
+        id
         jobs(securityReportTypes: $reportTypes) {
           nodes {
             name

changelogs/unreleased/326071-fix-security-mr-widget.yml

+---
+title: Fix security report fetching in Merge Requests
+merge_request: 57574
+author:
+type: fixed