[8.19] [ci] Only use cached es snapshot on pull requests (#268315) (#268534)

# Backport

This will backport the following commits from `main` to `8.19`:
- [[ci] Only use cached es snapshot on pull requests
(#268315)](#268315)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT
[{"author":{"name":"Jon","email":"jon@elastic.co"},"sourceCommit":{"committedDate":"2026-05-08T18:51:31Z","message":"[ci]
Only use cached es snapshot on pull requests (#268315)\n\nES snapshots
should trigger a cache update within an hour after
an\nupgrade.\n\nDuring the interim period, or if there are cache update
failures, we're\nfinding the parallelized number of download requests
becoming\nproblematic. If there's a cache update issue, it will be fixed
with the\nsame priority as a snapshot
build.","sha":"5d3bce05af431ab374d8db7748a4158a98a1f547","branchLabelMapping":{"^v9.5.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Team:Operations","release_note:skip","backport:all-open","v9.5.0"],"title":"[ci]
Only use cached es snapshot on pull
requests","number":268315,"url":"https://github.com/elastic/kibana/pull/268315","mergeCommit":{"message":"[ci]
Only use cached es snapshot on pull requests (#268315)\n\nES snapshots
should trigger a cache update within an hour after
an\nupgrade.\n\nDuring the interim period, or if there are cache update
failures, we're\nfinding the parallelized number of download requests
becoming\nproblematic. If there's a cache update issue, it will be fixed
with the\nsame priority as a snapshot
build.","sha":"5d3bce05af431ab374d8db7748a4158a98a1f547"}},"sourceBranch":"main","suggestedTargetBranches":[],"targetPullRequestStates":[{"branch":"main","label":"v9.5.0","branchLabelMappingKey":"^v9.5.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/268315","number":268315,"mergeCommit":{"message":"[ci]
Only use cached es snapshot on pull requests (#268315)\n\nES snapshots
should trigger a cache update within an hour after
an\nupgrade.\n\nDuring the interim period, or if there are cache update
failures, we're\nfinding the parallelized number of download requests
becoming\nproblematic. If there's a cache update issue, it will be fixed
with the\nsame priority as a snapshot
build.","sha":"5d3bce05af431ab374d8db7748a4158a98a1f547"}}]}]
BACKPORT-->

Co-authored-by: Jon <jon@elastic.co>

Author: kibanamachine

.buildkite/pipelines/pull_request/base.yml

@@ -1,3 +1,6 @@
+env:
+  KBN_ES_SNAPSHOT_USE_CACHED: 'true'
+
 steps:
   - command: .buildkite/scripts/steps/build_kibana.sh
     label: Build Kibana Distribution

src/platform/packages/shared/kbn-es/src/artifact.test.js

@@ -90,7 +90,11 @@ const createCachedArtifact = ({ contents, etag = 'etag' }) => {
 };
 
 const previousEnvVars = {};
-const ENV_VARS_TO_RESET = ['ES_SNAPSHOT_MANIFEST', 'KBN_ES_SNAPSHOT_USE_UNVERIFIED'];
+const ENV_VARS_TO_RESET = [
+  'ES_SNAPSHOT_MANIFEST',
+  'KBN_ES_SNAPSHOT_USE_UNVERIFIED',
+  'KBN_ES_SNAPSHOT_USE_CACHED',
+];
 
 beforeAll(() => {
   ENV_VARS_TO_RESET.forEach((key) => {

src/platform/packages/shared/kbn-es/src/artifact.ts

@@ -231,7 +231,7 @@ export class Artifact {
       const cacheMeta = cache.readMeta(dest);
       const tmpPath = `${dest}.tmp`;
 
-      if (useCached) {
+      if (useCached || process.env.KBN_ES_SNAPSHOT_USE_CACHED === 'true') {
         if (cacheMeta.exists) {
           this.log.info(
             'use-cached passed, forcing to use existing snapshot',