Improve vulnscan demo

[henrirosten]

Improve vulnscan demo:

• Rename vulnscan to vulnxscan
• setup.py: install entry point for vulnxscan
• Add flake target for vulnxscan
• vulnxscan.py: allow using SBOM as input

In addition, this PR adds the following fix:

• derivaiton.py: do not add drv store path as outpath