etc: remove unneeded temporary directions after putting in place the new etc

nixos/modules/system/etc/etc.nix

@@ -276,6 +276,22 @@ in
 
         # Unmount the top /etc mount to atomically reveal the new mount.
         umount --lazy --recursive /etc
+
+        # Unmount the temporary mount
+        umount --lazy "$tmpEtcMount"
+        rmdir "$tmpEtcMount"
+
+        # Unmount old metadata mounts
+        # For some reason, `findmnt /tmp --submounts` does not show the nested
+        # mounts. So we'll just find all mounts of type erofs and filter on the
+        # name of the mountpoint.
+        findmnt --type erofs --list --kernel --output TARGET | while read -r mountPoint; do
+          if [[ "$mountPoint" =~ ^/tmp/nixos-etc-metadata\..{10}$ &&
+                "$mountPoint" != "$tmpMetadataMount" ]]; then
+            umount --lazy $mountPoint
+            rmdir "$mountPoint"
+          fi
+        done
       fi
     '' else ''
       # Set up the statically computed bits of /etc.