Reviewer feedback: Use fetchPolicies constant

Uses `fetchPolicies` from `~/lib/graphql` instead of a hard-coded
string.

ee/app/assets/javascripts/vulnerabilities/components/vulnerability.vue

@@ -2,6 +2,7 @@
 import fetchHeaderVulnerabilityQuery from 'ee/security_dashboard/graphql/header_vulnerability.graphql';
 import { convertToGraphQLId } from '~/graphql_shared/utils';
 import { TYPE_VULNERABILITY } from '~/graphql_shared/constants';
+import { fetchPolicies } from '~/lib/graphql';
 import { s__ } from '~/locale';
 import createFlash from '~/flash';
 import { normalizeGraphQLVulnerability } from '../helpers';
@@ -38,7 +39,7 @@ export default {
     vulnerability: {
       manual: true,
       query: fetchHeaderVulnerabilityQuery,
-      fetchPolicy: 'no-cache',
+      fetchPolicy: fetchPolicies.NO_CACHE,
       variables() {
         return {
           id: convertToGraphQLId(TYPE_VULNERABILITY, this.vulnerability.id),