The libsodium-sys-stable build.rs panics when both SODIUM_LIB_DIR and
SODIUM_USE_PKG_CONFIG are set simultaneously. The runner environment at
172.0.0.29 has SODIUM_USE_PKG_CONFIG set (not traceable to any workflow
file or Docker image ENV layer), which conflicts with any step that sets
SODIUM_LIB_DIR.
For all three platform builds (linux-amd64, windows-amd64, linux-arm64):
- Add `unset SODIUM_USE_PKG_CONFIG` to the shell before cargo runs
- Switch Linux builds from inline apt-get to SODIUM_LIB_DIR (Docker
images already include libsodium-dev, removing the redundant install)
- Extend arm64 PKG_CONFIG_PATH to include /usr/aarch64-linux-gnu/lib/pkgconfig
(matching the beta workflow for consistency)
The unset is a no-op when the variable is absent; it cleanly clears any
ambient value injected by the runner infrastructure.
