[microvm] share read only nix store

this reduces build times drastically

machines/configuration.nix

@@ -46,6 +46,11 @@ let
     {
       microvm = {
         hypervisor = "qemu";
+        shares = [ {
+          tag = "ro-store";
+          source = "/nix/store";
+          mountPoint = "/nix/.ro-store";
+        } ];
         interfaces = [
           {
             type = "tap";