fix(boot): cap the systemd-boot entries at ten per host

Without configurationLimit every generation keeps its entry on the ESP.
The desktop had grown to 58 of them, 232M of a 1G partition, and nothing
ever removed one. Ten keeps a useful rollback range and lets the builder
clear the rest on every switch.

hosts/testvm/default.nix

@@ -29,6 +29,7 @@
   };
 
   boot.loader.systemd-boot.enable = true;
+  boot.loader.systemd-boot.configurationLimit = 10;
   boot.loader.efi.canTouchEfiVariables = true;
   boot.kernelParams = [ "quiet" ];