nixos/wpa_supplicant: ignore shellcheck nitpick

This can occur if one enables `systemd.enableStrictShellChecks`.

Shellcheck itself writes on this notice that one should use arrays in
this situation. I tried to do that, but the curly braces with nix
variable expansion gets boring very quickly. As such, we just ignore
this nitpick, as the use is reasonable here.
https://www.shellcheck.net/wiki/SC2086

Also enable `enableStrictShellChecks` to avoid regressing.

nixos/modules/services/networking/wpa_supplicant.nix

@@ -183,8 +183,10 @@ let
         }
 
         # finally start daemon
+        # shellcheck disable=SC2086
         exec wpa_supplicant $args
       '';
+      enableStrictShellChecks = true;
     };
 
   systemctl = "/run/current-system/systemd/bin/systemctl";