nixos/openvpn: path now requires conversion to a string

Following changes in NixOS/nixpkgs#91092 the `path` attribute is now a list
instead of being a string. This resulted resulted in the following evaluation error:

"cannot coerce a list to a string, at [...]/nixos/modules/services/networking/openvpn.nix:16:18"

so we now need to convert it to the right type ourselves.

Closes NixOS/nixpkgs#97360.

nixos/modules/services/networking/openvpn.nix

@@ -11,7 +11,7 @@ let
   makeOpenVPNJob = cfg: name:
     let
 
-      path = (getAttr "openvpn-${name}" config.systemd.services).path;
+      path = makeBinPath (getAttr "openvpn-${name}" config.systemd.services).path;
 
       upScript = ''
         #! /bin/sh