Enhancement: Add SLSA provenance to GHCR container images

Restic's GitHub Container Registry (GHCR) image build workflow now includes
SLSA (Supply-chain Levels for Software Artifacts) provenance generation.

Please see the restic documentation for more information about verifying SLSA
provenance.

https://github.com/restic/restic/issues/4983
https://github.com/restic/restic/pull/4999
