Revert "nixos/qemu-vm: replace postBootCommands by a systemd service" (#496623)

nixos/modules/virtualisation/qemu-vm.nix

@@ -1236,25 +1236,11 @@ in
     # allow `system.build.toplevel' to be included.  (If we had a direct
     # reference to ${regInfo} here, then we would get a cyclic
     # dependency.)
-    systemd.services.register-nix-paths = lib.mkIf config.nix.enable {
-      wantedBy = [
-        "multi-user.target"
-      ];
-      after = [
-        "nix-daemon.service"
-        "nix-daemon.socket"
-      ];
-      restartIfChanged = false;
-      serviceConfig = {
-        Type = "oneshot";
-        RemainAfterExit = true;
-      };
-      script = ''
-        if [[ "$(cat /proc/cmdline)" =~ regInfo=([^ ]*) ]]; then
-          ${config.nix.package.out}/bin/nix-store --load-db < "''${BASH_REMATCH[1]}"
-        fi
-      '';
-    };
+    boot.postBootCommands = lib.mkIf config.nix.enable ''
+      if [[ "$(cat /proc/cmdline)" =~ regInfo=([^ ]*) ]]; then
+        ${config.nix.package.out}/bin/nix-store --load-db < ''${BASH_REMATCH[1]}
+      fi
+    '';
 
     boot.initrd.availableKernelModules =
       optional (cfg.qemu.diskInterface == "scsi") "sym53c8xx" ++ optional (cfg.tpm.enable) "tpm_tis";