pkgs.writeErofsLayers,profiles/ch-runner: MVE

$ nix-build -A examples.dummy.config.debug.closure.erofs.list | xargs
cat | xargs du -h
749M    /nix/store/bzfv5x6lycq6hzhjv6d6vlk1q8fdg9di-base0.erofs
24M     /nix/store/hp41jfq36y0mmjrzqilyh3jfsvqic3kb-nixos.erofs

$ nix run -f . examples.dummy.config.uvms.cloud-hypervisor.runner
...
<<< Welcome to NixOS 25.11pre-git (x86_64) - ttyS0 >>>

nixos login:

The definition of the `pkgs` fixpoint was moved to pkgs/default.nix.
For that, dirToAttrs was moved to lib/, imported ad hoc

profiles/vsock-connect-guest.nix

@@ -1,18 +1,27 @@
-{ options, lib, ... }:
 {
-  config =
-    lib.optionalAttrs (options ? "microvm") {
+  options,
+  config,
+  lib,
+  ...
+}:
+let
+  mkIfGuest = import ../lib/mkIfMicrovmGuest.nix { inherit options config lib; };
+in
+{
+  config = lib.mkMerge [
+    (mkIfGuest {
       microvm.cloud-hypervisor.extraArgs = [
         "--vsock"
         "cid=4,socket=CONNECT.sock"
       ];
-    }
-    // {
+    })
+    {
       # Somehow, sshd calls to PAM with PAM_RHOST="UNKNOWN",
       # prompting a slow DNS look-up each time...
       #
       # https://mastodon.acm.org/@nobody/115108458851355328
       # https://github.com/linux-pam/linux-pam/issues/885#issuecomment-3030698895
       networking.hosts."100::" = [ "UNKNOWN" ];
-    };
+    }
+  ];
 }