fix(release): fall back when minisign apt package is unavailable

[wgqqqqq]

Summary

• fall back to installing minisign with Cargo when the runner apt repository does not provide it
• emit a clear error when neither minisign nor Cargo is available
• provision Rust in desktop and nightly publishing jobs so the fallback is available there too

This fixes signed Linux archive publishing on ubuntu-24.04-arm, where apt-get install minisign currently fails with Unable to locate package minisign. Fork builds without signing secrets continue to skip signing as before.

Verification

• bash -n scripts/sign-release-assets.sh
• pnpm run check:github-config
• node --test scripts/relay/package-contract.test.mjs scripts/check-github-config.test.mjs
• git diff --check