use serverless-specific wheel in layer

[emmettbutler]

What does this PR do?

Motivation

Testing Guidelines

DataDog/dd-trace-py#17201

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)

[litianningdatadog]

@emmettbutler , could you

1. fill in the PR comment
2. confirm ddtrace_serverless wheel is already being published to S3 by the upstream pipeline before merging
3. move this into "ready to review" status