Replace deprecated legacy editable install of db

[chuckwondo]

During build/deploy, we see the following deprecation warning, which we want to rectify prior to breakage in pip 25.0:

DEPRECATION: Legacy editable install of db==0.0.1 from file:///home/runner/work/hls-sentinel2-downloader-serverless/hls-sentinel2-downloader-serverless/layers/db (setup.py develop) is deprecated. pip 25.0 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at pypa/pip#11457