[ci] Trigger VM image rebuild for cache rebuild only (#213497)

## Summary
When VM image rebuild is triggered after ES promotion, only the cache
warmup should be built.

This PR also separates the daily full build to a daily base + cache
build (in case ES promotions are failing for some reason, we should
still have a daily cache refresh).

Requires: https://github.com/elastic/ci-agent-images/pull/1295

With this, we'd run a daily base image build and cache build (~40m +
25m) + cache warmups for every promotion (~4x 25m) instead of a full
build and promotion per build (~4x 55m). Ultimately not that much of a
gain 🤷 (4*55=220m => 40+5x25=165m)

.buildkite/pipeline-resource-definitions/kibana-vm-images.yml

@@ -27,17 +27,24 @@ spec:
       provider_settings:
         trigger_mode: none
       schedules:
-        daily kibana image build:
+        daily kibana base image build:
           branch: main
           cronline: '0 0 * * *'
           env:
-            IMAGES_CONFIG: kibana/images.yml
-          message: Builds Kibana VM images daily
+            IMAGES_CONFIG: kibana/base_image.yml
+          message: Builds Kibana VM base image daily
+        daily kibana cache layer build:
+          branch: main
+          cronline: '0 1 * * *' # make sure this runs after the daily kibana base image build
+          env:
+            IMAGES_CONFIG: kibana/image_cache.yml
+            BASE_IMAGES_CONFIG: 'core/images.yml,kibana/base_image.yml'
+          message: Builds Kibana VM cache warmup daily
         daily kibana fips image build:
           branch: main
-          cronline: '0 4 * * *' # make sure this runs after the daily kibana image build
+          cronline: '0 4 * * *' # make sure this runs after the daily kibana cache image build
           env:
-            BASE_IMAGES_CONFIG: 'core/images.yml,kibana/images.yml'
+            BASE_IMAGES_CONFIG: 'core/images.yml,kibana/base_image.yml,kibana/image_cache.yml'
             IMAGES_CONFIG: kibana/fips.yml
           message: Builds Kibana FIPS VM image daily
       teams:

.buildkite/scripts/steps/es_serverless/promote_es_serverless_image.sh

@@ -77,6 +77,7 @@ steps:
     async: true
     build:
       env:
-        IMAGES_CONFIG: "kibana/images.yml"
+        IMAGES_CONFIG: 'kibana/image_cache.yml'
+        BASE_IMAGES_CONFIG: 'core/images.yml,kibana/base_image.yml'
         RETRY: "1"
 EOF

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

@@ -25,6 +25,7 @@ steps:
     async: true
     build:
       env:
-        IMAGES_CONFIG: "kibana/images.yml"
+        IMAGES_CONFIG: 'kibana/image_cache.yml'
+        BASE_IMAGES_CONFIG: 'core/images.yml,kibana/base_image.yml'
         RETRY: "1"
 EOF

.buildkite/scripts/steps/fleet/promote_package_registry.sh

@@ -14,7 +14,8 @@ steps:
     async: true
     build:
       env:
-        IMAGES_CONFIG: "kibana/images.yml"
+        IMAGES_CONFIG: 'kibana/image_cache.yml'
+        BASE_IMAGES_CONFIG: 'core/images.yml,kibana/base_image.yml'
         RETRY: "1"
 EOF
 else