Adds TAURI_SIGNING_PRIVATE_KEY(_PASSWORD) env to release-linux, release-linux-aarch64, and release-macos build steps, collects the resulting .sig files into the release artifacts, and adds a new release-updater-manifest workflow that waits for all platform sig files to land on the release and assembles latest.json for the updater endpoint. Cherry-picked from #305 by @afonsojramos. cc