fix: scope fstab fallback mode to non-unix builds

traffic_core/src/fstab.rs

@@ -21,8 +21,8 @@ use std::{
 use crate::atomic;
 
 pub const FSTAB_PATH: &str = "/etc/fstab";
-/// Used only when the live file's mode cannot be read, which on Linux means it is about
-/// to fail for other reasons anyway.
+/// Fallback for non-Unix test builds, where Unix permission bits are unavailable.
+#[cfg(not(unix))]
 const FSTAB_FALLBACK_MODE: u32 = 0o644;
 
 #[cfg(unix)]