[bugfix] Remove passing CI container registry credentials into inspector-sbomgen by default

When INSPECTOR_SBOMGEN_USERNAME or INSPECTOR_SBOMGEN_PASSWORD are set when using inspector-sbomgen container --image, sbomgen will attempt to use these credentials when pulling from any container registry, including public registries such as ECR Public Gallery, or Dockerhub.

Customers should instead set INSPECTOR_SBOMGEN_USERNAME or INSPECTOR_SBOMGEN_PASSWORD as a part of their CI variables, when needed.