GHCR stores images with lowercase names only. The SBOM action was using
the mixed-case github.repository value which caused Syft to fail when
trying to pull the image.
Add IMAGE_NAME_LC environment variable with lowercase image name
Update SBOM action, Trivy scan, and docker commands to use lowercase
Applied to all jobs: build-and-push-nightly, test-nightly-image,
verify-nightly-supply-chain
Fixes nightly-build.yml workflow failure in "Generate SBOM" step