[9.1] (backport #10943) fix: pass all necessary env vars so retried fips steps provision ESS

[mergify]

What does this PR do?

This PR adds the required environment variables (EC_ENDPOINT and ESS_REGION) to all FIPS integration test steps in the Buildkite pipeline to ensure proper ESS (Elastic Stack Service) stack provisioning during test retrying.

Why is it important?

Without these environment variables, retried FIPS integration test steps fail to provision ESS stacks correctly, causing test failures. This issue was discovered when investigating this failed build, where every retried FIPS step failed ESS stack provisioning due to the missing EC_ENDPOINT configuration (at least this is my working theory).

Interestingly, retried fips:x86_64:sudo-false:fleet CI steps appeared successful, but further investigation revealed they don't actually run any tests, which masked the underlying issue.

This fix ensures that all FIPS integration tests have the correct environment configuration regardless of whether they are initial runs or retried steps.

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

No disruptive user impact. This change only affects the CI pipeline configuration and does not modify any user-facing functionality.

How to test this PR locally

This change affects the Buildkite CI pipeline configuration.

Related issues

N/A

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This is an automatic backport of pull request #10943 done by Mergify.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 21fc6f9

cc @pkoutsovasilis