PR #105 (beta) and PR #106 (master) independently fixed libsodium detection
using different approaches, causing conflicts on every auto master→beta merge.
Adopt beta's proven approach in auto-tag.yml and release-beta.yml:
- linux-amd64: use SODIUM_LIB_DIR env var, remove runtime apt-get install
- linux-arm64: use SODIUM_LIB_DIR / extended PKG_CONFIG_PATH, remove apt-get
- windows: keep SODIUM_STATIC, add SODIUM_USE_PKG_CONFIG=no