nixos/drbd: fix

- fix environment.etc."drbd.conf"
- don't generate an ExecStart script for just one command

nixos/modules/services/network-filesystems/drbd.nix

···
         options drbd usermode_helper=/run/current-system/sw/bin/drbdadm
       '';
 
-    environment.etc.drbd.conf =
+    environment.etc."drbd.conf" =
       { source = pkgs.writeText "drbd.conf" cfg.config; };
 
     systemd.services.drbd = {
       after = [ "systemd-udev.settle.service" "network.target" ];
       wants = [ "systemd-udev.settle.service" ];
       wantedBy = [ "multi-user.target" ];
-      script = ''
-        ${pkgs.drbd}/sbin/drbdadm up all
-      '';
-      serviceConfig.ExecStop = ''
-        ${pkgs.drbd}/sbin/drbdadm down all
-      '';
+      serviceConfig = {
+        ExecStart = "${pkgs.drbd}/sbin/drbdadm up all";
+        ExecStop = "${pkgs.drbd}/sbin/drbdadm down all";
+      };
     };
   };
 }