Tested locally with Ubuntu 22.04 + libsodium-sys-stable 1.24.0:
- No env vars → install_from_source() needs 'make', fails in slim containers
- SODIUM_LIB_DIR alone → works when libsodium is installed in the image
- SODIUM_LIB_DIR + SODIUM_USE_PKG_CONFIG → immediate panic (incompatible by design)
Fixes three broken build targets introduced by the merge conflict resolution:
- linux-amd64: was missing all sodium config, add SODIUM_LIB_DIR
- windows: had SODIUM_LIB_DIR + SODIUM_USE_PKG_CONFIG (incompatible), remove the latter
- linux-arm64: was missing sodium config, add SODIUM_LIB_DIR