Fix/renovate pipeline (elastic#204672)

## Summary

Renovate pipeline isn't being uploaded to Buildkite properly and `pre`
and `post` build steps were not necessary and create errors with CI
stats.

[Successful CI
run](https://buildkite.com/elastic/kibana-pull-request/builds/261627)

(cherry picked from commit ecd4567)

.buildkite/pipelines/pull_request/renovate.yml

@@ -1,12 +1,4 @@
 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:

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

@@ -39,15 +39,16 @@ const getPipeline = (filename: string, removeSteps = true) => {
       return;
     }
 
+    pipeline.push(getAgentImageConfig({ returnYaml: true }));
+
     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.');
+
+      console.log([...new Set(pipeline)].join('\n'));
       return;
     }
 
-    pipeline.push(getAgentImageConfig({ returnYaml: true }));
     pipeline.push(getPipeline('.buildkite/pipelines/pull_request/base.yml', false));
 
     if (await doAnyChangesMatch([/^packages\/kbn-handlebars/])) {