v1.4.0: sbomnix: add support for sbom output in spdx json format

- sbomnix: support spdx json output (--spdx) argument
- test: add relevant test cases to validate spdx output
- include spdx documents from sbomnix itself to release assets
- fix relevant documentation

In addition this commit includes a fix to Makefile to prune
python eggs from the list of python targets.

Signed-off-by: Henri Rosten <henri.rosten@unikie.com>