ci: install dd-trace-py pipeline wheels when run as child pipeline

[brettlangdon]

What does this PR do?

Update build layer scripts to pull dd-trace-py CI pipeline artifacts when this CI is run as a child pipeline of the dd-trace-py project.

Motivation

This ensures this CI is run with the pipeline wheels instead of defaulting to latest of PyPI. This will give us better detection of changes/PRs in dd-trace-py and their impact on this project.

Testing Guidelines

Additional Notes

We no longer build/ship/support Python 3.8 wheels for dd-trace-py, so there is a fallback mechanism to not fail if we cannot find the correct wheel to download.

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)