by default, don't use a tmpfs for the /nix/store in the VM tests

tests/module-allow-rebuild-in-vm.nix

@@ -1,4 +1,4 @@
-{ name, config, ... }:
+{ name, lib, config, ... }:
 
 {
   system.includeBuildDependencies = true;
@@ -12,6 +12,12 @@
     mountHostNixStore = true;
     installBootLoader = true;
 
+    # Moderately improves performance of `nixos-rebuild` inside the VM on my machine,
+    # (205.48 seconds to 186.42 seconds), but we have to allocate much more space.
+    writableStoreUseTmpfs = false;
+    # 20 GiB ought to be enough.
+    diskSize = lib.mkIf (!config.virtualisation.writableStoreUseTmpfs) (20 * 1024);
+
     # With how much memory Nix eval uses, this is essentially required.
     memorySize = 8192;
     cores = 8;