nixos/openvpn: use writeShellScript

Previously the upScript would fail with

Syntax error: "(" unexpected (expecting "done")

on line 7 if /bin/sh is not bash.

nixos/modules/services/networking/openvpn.nix

···
       path = makeBinPath (getAttr "openvpn-${name}" config.systemd.services).path;
 
       upScript = ''
-        #! /bin/sh
         export PATH=${path}
 
         # For convenience in client scripts, extract the remote domain
···
       '';
 
       downScript = ''
-        #! /bin/sh
         export PATH=${path}
         ${optionalString cfg.updateResolvConf
            "${pkgs.update-resolv-conf}/libexec/openvpn/update-resolv-conf"}
···
           ${optionalString (cfg.up != "" || cfg.down != "" || cfg.updateResolvConf) "script-security 2"}
           ${cfg.config}
           ${optionalString (cfg.up != "" || cfg.updateResolvConf)
-              "up ${pkgs.writeScript "openvpn-${name}-up" upScript}"}
+              "up ${pkgs.writeShellScript "openvpn-${name}-up" upScript}"}
           ${optionalString (cfg.down != "" || cfg.updateResolvConf)
-              "down ${pkgs.writeScript "openvpn-${name}-down" downScript}"}
+              "down ${pkgs.writeShellScript "openvpn-${name}-down" downScript}"}
           ${optionalString (cfg.authUserPass != null)
               "auth-user-pass ${pkgs.writeText "openvpn-credentials-${name}" ''
                 ${cfg.authUserPass.username}