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

tests/README.md

@@ -0,0 +1,6 @@
+# Notes for running the tests
+
+The VM tests do Nix builds inside the VMs, so they have writable Nix stores.
+Writable Nix stores in NixOS VM tests are, by default, tmpfs, but to speed up IO for the in-VM `nixos-rebuild`s, `./module-allow-rebuild-in-vm.nix` instead sets `virtualisation.writableStoreUseTmpfs = false;`, and allocates a much larger root filesystem for the VM.
+Specifically, each VM test allocates 20 GiB.
+If you are running low on disk space, disable `virtualisation.writableStoreUseTmpfs` in `./module-allow-rebuild-in-vm`.

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;