refactor: use the new overlay fileSystems API

Author: songpola

nixos-modules/microvm/mounts.nix

@@ -59,7 +59,7 @@ lib.mkIf config.microvm.guest.enable {
     ) {
       "/nix/store" = {
         device = hostStore.mountPoint;
-        options = [ "bind" ];
+        options = [ "ro" "bind" ];
         neededForBoot = true;
       };
     }
@@ -72,7 +72,7 @@ lib.mkIf config.microvm.guest.enable {
       "/nix/.ro-store" = {
         device = roStoreDisk;
         fsType = storeDiskType;
-        options = [ "x-systemd.requires=systemd-modules-load.service" ];
+        options = [ "ro" "x-systemd.requires=systemd-modules-load.service" ];
         neededForBoot = true;
         noCheck = true;
       };