[ci] Update VM cache for each ES snapshot (#205119)

Currently we're rebuilding our cache after the ES snapshot on main has
been promoted. This makes an assumption that less frequent cache builds
will save minutes over the chance of a build failure on main causing
snapshots for all other passing branches to be out of date.

That assumption hasn't been correct so far, so I'm moving the trigger to
all branches.

.buildkite/scripts/steps/es_snapshots/promote.sh

@@ -17,3 +17,14 @@ if [[ "$BUILDKITE_BRANCH" == "main" ]]; then
   echo "--- Trigger agent packer cache pipeline"
   ts-node .buildkite/scripts/steps/trigger_pipeline.ts kibana-agent-packer-cache main
 fi
+
+cat << EOF | buildkite-agent pipeline upload
+steps:
+  - label: "Update cache for ES $BUILDKITE_BRANCH snapshot"
+    trigger: kibana-vm-images
+    async: true
+    build:
+      env:
+        IMAGES_CONFIG: "kibana/images.yml"
+        RETRY: "1"
+EOF