Revert pyproject.toml changes, refactor sed command

[rithikanarayan]

What does this PR do?

Returns pyproject.toml to state from . Refactors system-tests build layer Github workflow to move injection of dev dd-trace-py wheel into /scripts/build_layers.sh using an environment variable.

Motivation

Deduplication of dd-trace-py wheel changing logic across datadog-lambda-python, serverless-tools, and dd-trace-py.

Testing Guidelines

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)