Get packages to build only from commit context

[cbix]

Currently the CI checks out the specific commit for that run and compares all package versions to the current versions in the package database. This causes several issues:

• pipelines are not reproducible as their context depends on external information in the live database
• restarting a successful pipeline won't build any package
• multiple parallel pipelines have undefined behavior and often build unnecessary packages

[cbix]

This was partially addressed in #324 but we're still mixing up responsibilities:

• git diff should determine packages to build
• db check should determine packages to upload