SBOM should only allow our default scanner image

[crazy-max]

Our reusable workflows current allow user to override the sbom generator image:

• github-builder-experimental/.github/workflows/build.yml

  Line 415 in 5481e60

  sbom: ${{ inputs.build-sbom }}

• github-builder-experimental/.github/workflows/bake.yml

  Line 509 in 5481e60

  sbom: ${{ inputs.bake-sbom }}

Our input should be a boolean to enable/disable sbom only.