nixos/slurm: fix startup of slurmd

* make slurmd depend on network target to ensure basic networking
  is available on startup. This fixes behaviour
  where slurmd fails with "error: get_addr_info: getaddrinfo() failed".
* Use tmpfiles.d to ensure spool directory exists on start up.

(cherry picked from commit 270da0a115)

nixos/modules/services/computing/slurm/slurm.nix

@@ -354,6 +354,7 @@ in
 
       wantedBy = [ "multi-user.target" ];
       after = [ "systemd-tmpfiles-clean.service" ];
+      requires = [ "network.target" ];
 
       serviceConfig = {
         Type = "forking";
@@ -363,12 +364,12 @@ in
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
         LimitMEMLOCK = "infinity";
       };
-
-      preStart = ''
-        mkdir -p /var/spool
-      '';
     };
 
+    systemd.tmpfiles.rules = mkIf cfg.client.enable [
+      "d /var/spool/slurmd 755 root root -"
+    ];
+
     services.openssh.forwardX11 = mkIf cfg.client.enable (mkDefault true);
 
     systemd.services.slurmctld = mkIf (cfg.server.enable) {