Add multi-arch GitHub CI packaging artifacts (amd64 + arm64)

[Copilot]

This PR adds GitHub CI logic to produce Linux binary packages for both amd64 and arm64 and publish them as workflow artifacts. Packaging is now automated per architecture instead of relying on manual local commands.

• CI workflow for package production

  • Added .github/workflows/build-packages.yml
  • Triggers on push (main/master), pull_request, and workflow_dispatch
  • Uses a matrix build for amd64 and arm64

• Per-arch binary + package build

  • Cross-compiles country_lockdown with GOOS=linux / GOARCH=<matrix arch>
  • Downloads matching gobgp/gobgpd release tarball for each arch
  • Builds both deb and rpm packages via nfpm

• Artifact publication

  • Uploads generated packages as per-arch artifacts:
    • country-lockdown-packages-amd64
    • country-lockdown-packages-arm64

• Workflow hardening and maintainability

  • Added explicit job permissions (contents: read, actions: write)
  • Centralized gobgp version via workflow env
  • Generates temporary arch-specific nfpm config in $RUNNER_TEMP before packaging

strategy:
  matrix:
    arch: [amd64, arm64]

- run: CGO_ENABLED=0 GOOS=linux GOARCH=${{ matrix.arch }} go build -mod=vendor -o bin/country_lockdown .
- run: ~/go/bin/nfpm pkg --config "${RUNNER_TEMP}/nfpm.yaml" --packager deb --target dist/
- uses: actions/upload-artifact@v4
  with:
    name: country-lockdown-packages-${{ matrix.arch }}
    path: dist/*