AMDSEV: harden reproducible VM component builds

[kariy]

• Require explicit SOURCE_DATE_EPOCH and remove wall-clock build metadata so host time cannot perturb outputs.
• Add optional apt snapshot source pinning (while keeping version+SHA checks) to eliminate package mirror drift as a hidden input.
• Enforce pinned OVMF_COMMIT and validate submodule state so firmware source resolution is immutable.
• Emit deterministic provenance (build-info.txt, materials.lock, INPUT_MANIFEST_SHA256, toolchain/container metadata) so rebuilds are auditable and comparable.
• Add machine-checkable verification (build.sh --repro-check, verify-build.sh --compare) so reproducibility is enforced by automated hash equivalence.