bk: retry Start ESS stack for integration tests

[v1v]

What does this PR do?

Retry to add some resilience when retrieving components from the internet.
Remove unnecessary shell leftovers.

Why is it important?

This should help with transitive flakiness:

• [8.17] (backport #8263) ci(serverless): use OIDC for running the serverless ITs #8515 (comment)

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: 6c4703e

Failed CI Steps

• Merge coverage reports

History

cc @v1v

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[v1v]

See https://buildkite.com/elastic/elastic-agent/builds/22589#01977e05-4825-4d3c-ad52-43d126df92ae/111-112: