Add SBOM and provenance generation

[shashial]

This commit generates a CycloneDX 1.6 SBOM from the mkosi manifest at build time;

Then in the "publish" workflow:

1. generates combined SHA256SUMS for artefacts incuding SBOM
2. signs SLSA provenance for all production artifacts plus an SBOM attestation for the .efi
3. verifies both bundles before upload
4. publishes manifest, SBOM, checksums and attestation bundles to R2

[Copilot]

Pull request overview

This PR adds supply-chain metadata generation and publishing for Flashbox L1 builds by producing a CycloneDX 1.6 SBOM from the mkosi manifest and signing/verifying provenance + SBOM attestations during the publish workflow.

Changes:

• Add an mkosi post-output script to generate a deterministic CycloneDX 1.6 SBOM from the mkosi JSON manifest.
• Extend the build/publish workflow to generate combined SHA256SUMS, create Sigstore (SLSA provenance + SBOM) attestations, verify them, and upload artifacts + bundles to R2 / attach to releases.
• Update mkosi module configuration to run shared post-output scripts; ignore Python __pycache__ artifacts.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
shared/mkosi.postoutput.d/91-deb-sbom.py	New Python post-output script that converts mkosi manifests into a deterministic CycloneDX 1.6 SBOM.
modules/flashbox/flashbox-l1/mkosi.conf	Enables mkosi post-output scripts for flashbox-l1 builds.
.gitignore	Ignores Python __pycache__/ directories.
.github/workflows/flashbox-l1.yaml	Adds permissions needed to mint/verify attestations in the release job.
.github/workflows/_build-and-publish-image.yaml	Adds checksum generation, provenance/SBOM attestation creation, verification, and publishing of SBOM + bundles.

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[alexhulbert]

Generally looks good except the python file looks a bit overengineered, the shared folder has some flashbots-specific stuff (other companies use it shared folder so we need to keep it generic), and ive run into reproducibility issues with python-based post output files in the past.

ill write up a small ~30-40 line jq script when i get back from my vacation on monday and you lmk if that's a suitable alternative to the python one you have here.