ci(fix): refactor serverless-system-tests-build-layer (#16348)

## Description
The definition of the ddtrace dependency in datadog-lambda-python is
being modified and the build_layers.sh script in that repo is being
refactored to deduplicate the use of the sed command injecting the
ddtrace wheel in different repos
[PR](DataDog/datadog-lambda-python#720). This PR
brings dd-trace-py into compatibility with those changes.
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## Testing

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## Risks

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## Additional Notes

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Co-authored-by: Florentin Labelle <florentin.labelle@datadoghq.com>

Author: rithikanarayan

.github/workflows/system-tests.yml

@@ -384,8 +384,7 @@ jobs:
       - name: Build datadog_lambda layer
         run: |
           wheel_path=$(find ./artifacts -name "*.whl" | head -n 1)
-          sed -i 's|^ddtrace =.*$|ddtrace = { file = "'"$wheel_path"'" }|' pyproject.toml
-          ARCH=amd64 PYTHON_VERSION=3.13 ./scripts/build_layers.sh
+          ARCH=amd64 PYTHON_VERSION=3.13 DD_TRACE_WHEEL=$wheel_path ./scripts/build_layers.sh
 
       - name: Upload layer artifact
         uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0