test-vm: Increase disk size

hosts/test-vm/configuration.nix

@@ -26,9 +26,9 @@
     security.sudo.wheelNeedsPassword = false;
 
     virtualisation.vmVariant = {
-      # following configuration is added only when building VM with build-vm
+      # following configuration is added only when building VM the *first* time with `build-vm`
       virtualisation = {
-        diskSize = 4096;
+        diskSize = 8192;
         memorySize = 2048;
         cores = 3;
         graphics = false;