ci: fix serverless system-tests build layer

[florentinl]

What does this PR do?

This PR fixes the sed command in the build_layers.yml job. The sed command is in multiline mode (-z) so matching ^ddtrace = .*$ matches only ddtrace = at the start of the file and thus does nothing. Therefore, the resulting build contains the released version of dd-trace-py instead of the dev one.

Motivation

Testing Guidelines

Looking at job logs, the wheel is correctly picked up:

Additional Notes

Types of Changes

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

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• 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
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