Only run quick checks for changes to renovate.json (elastic#203153)

## Summary

Introduces a new `renovate` CI pipeline to only run quick checks for PRs
which only adjust the `renovate.json` configuration file.

.buildkite/pipelines/pull_request/renovate.yml

@@ -0,0 +1,20 @@
+steps:
+  - command: .buildkite/scripts/lifecycle/pre_build.sh
+    label: Pre-Build
+    timeout_in_minutes: 10
+    agents:
+      machineType: n2-standard-2
+
+  - wait
+
+  - command: .buildkite/scripts/steps/renovate.sh
+    label: 'Renovate validation'
+    agents:
+      machineType: n2-highcpu-8
+      preemptible: true
+    key: renovate_validation
+    timeout_in_minutes: 60
+    retry:
+      automatic:
+        - exit_status: '-1'
+          limit: 3

.buildkite/scripts/pipelines/pull_request/pipeline.ts

@@ -39,6 +39,14 @@ const getPipeline = (filename: string, removeSteps = true) => {
       return;
     }
 
+    const onlyRunQuickChecks = await areChangesSkippable([/^renovate\.json$/], REQUIRED_PATHS);
+    if (onlyRunQuickChecks) {
+      pipeline.push(getPipeline('.buildkite/pipelines/pull_request/renovate.yml', false));
+      pipeline.push(getPipeline('.buildkite/pipelines/pull_request/post_build.yml'));
+      console.log('Isolated changes to renovate.json. Skipping main PR pipeline.');
+      return;
+    }
+
     pipeline.push(getAgentImageConfig({ returnYaml: true }));
     pipeline.push(getPipeline('.buildkite/pipelines/pull_request/base.yml', false));
 

.buildkite/scripts/steps/renovate.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+echo '--- Renovate: validation'
+.buildkite/scripts/steps/checks/renovate.sh