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>

v1.3.0: Upload sbomnix sbom to github release artifacts

- Upload runtime and buildtime SBOMs from sbomnix itself to
  release artifacts on every push of a new release tag
- Up the sbomnix version to v1.3.0

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

Release v1.2.0

Up the version to 1.2.0

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

Release v1.1.0

Merge pull request #34 from henrirosten/fixes_from_manually_testing_o…

Release v1.0.0

Merge pull request #25 from henrirosten/readme_fix_toc

doc: fix toc in main readme