virtualisation/azure-image: use mkImageMediaOverride...

...for filesystem options. Before this change,
users would typically encounter conflicting option definitions
when trying to build an image for a generic nixos closure, i.e.
`nixos-rebuild build-image --image-variant azure --flake .#my-host`

nixos/modules/virtualisation/azure-image.nix

@@ -113,7 +113,7 @@ in
       '';
     };
 
-    fileSystems = {
+    fileSystems = lib.mkImageMediaOverride {
       "/" = {
         device = "/dev/disk/by-label/${cfg.label}";
         inherit (cfg) label;