[Scout] Upload screenshots for failed tests to buildkite artifacts (elastic#236613)

## Summary

While investigating a test failure, I've found that Scout doesn't upload
captured screenshots for failed tests to Buildkite Artifacts.

### Before
For example for the following failure:

<img width="3456" height="1924" alt="image"
src="https://github.com/user-attachments/assets/c947c834-dd67-465e-8f09-fa5a1b356ede"
/>

Playwright did capture screenshot
```shell

proc [playwright]     attachment #1: screenshot (image/png) ──────────────────────────────────────────────────────────
--
  | proc [playwright]     x-pack/platform/plugins/shared/streams_app/test/scout/ui/.scout/test-artifacts/data_management-data_proce-582f5-during-a-processor-creation-local/test-failed-1.png
  | proc [playwright]     ────────────────────────────────────────────────────────────────────────────────────────────────
```
but it's not available under job's **Artifacts**
<img width="3456" height="1910" alt="image"
src="https://github.com/user-attachments/assets/027e16fb-7a08-4d7a-bd74-804690263fae"
/>

### After

#### **Failing test** (see
[build](https://buildkite.com/elastic/kibana-pull-request/builds/344414))

<img width="3442" height="1922" alt="image"
src="https://github.com/user-attachments/assets/558a6a10-f2b0-4491-a54d-cef89b827745"
/>

<img width="3442" height="1916" alt="image"
src="https://github.com/user-attachments/assets/0c1e080d-de40-4a7c-aff3-cab93d4a64c0"
/>

#### **Passing test**

<img width="3456" height="1906" alt="image"
src="https://github.com/user-attachments/assets/5e4f0171-60a0-45b4-a1ab-39fb94f935ce"
/>

---------

Co-authored-by: Elastic Machine <[email protected]>

Author: 2 people

.buildkite/scripts/lifecycle/post_command.sh

@@ -10,6 +10,7 @@ IS_TEST_EXECUTION_STEP="$(buildkite-agent meta-data get "${BUILDKITE_JOB_ID}_is_
 
 if [[ "$IS_TEST_EXECUTION_STEP" == "true" ]]; then
   echo "--- Upload Artifacts"
+  buildkite-agent artifact upload '**/.scout/test-artifacts/**/*.png'
   buildkite-agent artifact upload '.scout/reports/scout-playwright-test-failures-*/**/*'
   buildkite-agent artifact upload 'target/junit/**/*'
   buildkite-agent artifact upload 'target/kibana-coverage/jest/**/*'