grubconfig: should use /sysroot for the physical root

Get hints from bootc code https://github.com/bootc-dev/bootc/blob/main/crates/lib/src/install.rs#L2017

Author: HuijingHei

src/grubconfigs.rs

@@ -31,8 +31,10 @@ pub(crate) fn install(
     write_uuid: bool,
 ) -> Result<()> {
     let bootdir = &target_root.sub_dir("boot").context("Opening /boot")?;
+    // Should use /sysroot for the physical root
+    let rootdir = &target_root.sub_dir("sysroot").context("Opening /sysroot")?;
     let boot_is_mount = {
-        let root_dev = target_root.self_metadata()?.stat().st_dev;
+        let root_dev = rootdir.self_metadata()?.stat().st_dev;
         let boot_dev = bootdir.self_metadata()?.stat().st_dev;
         log::debug!("root_dev={root_dev} boot_dev={boot_dev}");
         root_dev != boot_dev