ci(system-tests): fix ddtrace py build from source

[florentinl]

What does this PR do?

Fix the build_layer.yml github action by adding a missing symlink for the c++ compiler

Motivation

Failing pipelines on main for AL2 images: https://github.com/DataDog/datadog-lambda-python/actions/runs/19683478639

Testing Guidelines

Triggered a github action from this branch to verify it works correctly: https://github.com/DataDog/datadog-lambda-python/actions/runs/19827184647

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[cbeauchesne]

Could you add this job to pull requests ? it will prevent a futur PR to break it

[florentinl]

/merge

[dd-devflow-routing-codex]

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2025-12-01 16:39:19 UTC ℹ️ Start processing command /merge

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2025-12-01 16:46:18 UTC ℹ️ MergeQueue: merge request added to the queue

The expected merge time in main is approximately 0s (p90).

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2025-12-01 16:54:10 UTC ℹ️ MergeQueue: This merge request was already merged

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