Cargo injects [env] entries directly into build script environments,
bypassing shell-level unset and env -u entirely. SODIUM_USE_PKG_CONFIG
was set to "0" in [env], which libsodium-sys-stable build.rs treats as
"pkg-config enabled" (env::var().is_ok() returns true for any value
including "0"). Combined with SODIUM_LIB_DIR set in the workflow, this
triggered the incompatibility panic on every build.
The original comment claiming this "avoids memset_explicit on Windows"
was incorrect — SODIUM_USE_PKG_CONFIG controls detection strategy, not
link behaviour. The actual Windows memset_explicit fix is handled via
SODIUM_LIB_DIR pointing to pre-built MinGW sodium.
