nixos/nix-optimise: be smarter about when we run the store optimiser

We might be inside a NixOS container on a non-NixOS host, so instead of not
running at all inside a container, check if the nix-daemon socket is writable as
it will tell us if the store is managed from here or outside.

Fixes #63578

nixos/modules/services/misc/nix-optimise.nix

@@ -40,8 +40,8 @@ in
 
     systemd.services.nix-optimise =
       { description = "Nix Store Optimiser";
-        # No point running it inside a nixos-container. It should be on the host instead.
-        unitConfig.ConditionVirtualization = "!container";
+        # No point this if the nix daemon (and thus the nix store) is outside
+        unitConfig.ConditionPathIsReadWrite = "/nix/var/nix/daemon-socket";
         serviceConfig.ExecStart = "${config.nix.package}/bin/nix-store --optimise";
         startAt = optionals cfg.automatic cfg.dates;
       };

nixos/modules/virtualisation/container-config.nix

@@ -7,6 +7,7 @@ with lib;
   config = mkIf config.boot.isContainer {
 
     # Disable some features that are not useful in a container.
+    nix.optimise.automatic = mkDefault false; # the store is host managed
     services.udisks2.enable = mkDefault false;
     powerManagement.enable = mkDefault false;