release: wire rustic to sagemathinc fork and linux releases

Point rustic at the sagemathinc/rustic_core fork so release builds and
self-update use the forked hardlink-preserving restore implementation.
This updates the repository metadata, lockfile, and self-update target
to reference the sagemathinc org instead of the upstream rustic-rs org.

Replace the upstream-only release workflow with a small tag-driven Linux
release pipeline for this fork. It publishes exactly two artifacts,
linux-x86_64 and linux-arm64, and attaches tarballs plus sha256 files to
a draft GitHub release.