Begin the nix store overlay

We need to disable the read-only bind mount, so we can directly bind
mount the overlay.

m/module/shared-nix-store.nix

@@ -0,0 +1,18 @@
+{ ... }:
+{
+  # Don't make the nix store read-only, as this would prevent the overlay FS
+  # from being able to mount it.
+  boot.readOnlyNixStore = false;
+
+  # Mount the hut nix store via NFS
+  fileSystems."/mnt/hut-nix-store" = {
+    device = "hut:/nix/store";
+    fsType = "nfs";
+    options = [ "ro" ];
+  };
+
+  # Create the work directory
+  #systemd.tmpfiles.rules = [
+  #  "d /mnt/nix-overlay/work 0550 1000 root root -"
+  #];
+}

m/owl1/configuration.nix

@@ -5,6 +5,7 @@
     ../common/main.nix
     ../module/ceph.nix
     ../module/slurm-firewall.nix
+    ../module/shared-nix-store.nix
   ];
 
   # Select the this using the ID to avoid mismatches

m/owl2/configuration.nix

@@ -5,6 +5,7 @@
     ../common/main.nix
     ../module/ceph.nix
     ../module/slurm-firewall.nix
+    ../module/shared-nix-store.nix
   ];
 
   # Select the this using the ID to avoid mismatches