ci(system-tests): avoid rebuilding ddtrace from source

[florentinl]

What does this PR do?

This PR improves the github action that builds layers for the system-tests by avoiding to rebuild ddtrace from source and fetching wheels from the dd-trace-py pipeline instead.

Motivation

• Reduce job duration from ~13-15mins to ~2mins

Testing Guidelines

Triggered the pipeline on this branch: https://github.com/DataDog/datadog-lambda-python/actions/runs/19826626595

Additional Notes

• dd-trace-py does not produce artifacts for python 3.8 and system-tests does not use the resulting build either, so I removed it

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)

[brettlangdon]

lgtm