stage-2-init: fix false positives for RO Nix store mounts

We need to take the "top" mount instead of any mount, which is the last
line printed by findmnt. Additionally, make the regex more strict, so we
don't select mount options ending in ro (like `errors=remount-ro` from
ext4, or overlay paths ending in 'ro') and accidentally leave the Nix
store RW after boot.

nixos/modules/system/boot/stage-2-init.sh

@@ -69,7 +69,8 @@ fi
 chown -f 0:30000 /nix/store
 chmod -f 1775 /nix/store
 if [ -n "@readOnlyNixStore@" ]; then
-    if ! [[ "$(findmnt --noheadings --output OPTIONS /nix/store)" =~ ro(,|$) ]]; then
+    # #375257: Ensure that we pick the "top" (i.e. last) mount so we don't get a false positive for a lower mount.
+    if ! [[ "$(findmnt --direction backward --first-only --noheadings --output OPTIONS /nix/store)" =~ (^|,)ro(,|$) ]]; then
         if [ -z "$container" ]; then
             mount --bind /nix/store /nix/store
         else