Add Ubuntu Homebrew LibreSSL CI validation for Linux compatibility

[daviduhden]

Summary

This PR adds Ubuntu CI coverage for building i2pd against Homebrew LibreSSL on Linux, alongside the existing OpenSSL-based matrix.

What changes

• updates .github/workflows/build.yml
• modernizes workflow hardening in existing jobs:
  • pins actions/checkout
  • scopes permissions to contents: read
  • disables credential persistence
• adds a new job: build-cmake-homebrew-libressl
  • sets up Homebrew on Ubuntu
  • installs LibreSSL from Homebrew
  • configures CMake to link against Homebrew LibreSSL
  • builds i2pd with UPnP enabled
  • runs a binary smoke test (i2pd --version) with LibreSSL runtime path

Why this is better

• confirms LibreSSL compatibility on Linux in CI, not only on OpenBSD/macOS
• provides an early compatibility signal for LibreSSL-specific regressions
• keeps existing Ubuntu build coverage intact while adding a targeted TLS-variant lane