nixos/kimai: set PHP package for PHP-FPM pool to Kimai's PHP package

I've already made it so that Kimai's PHP package has all required
extensions. So use that instead of the default PHP package.

This fixes a warning in Kimai's doctor page.

nixos/modules/services/web-apps/kimai.nix

@@ -238,6 +238,7 @@ in
       services.phpfpm.pools = mapAttrs' (
         hostName: cfg:
         (nameValuePair "kimai-${hostName}" {
+          phpPackage = cfg.package.php;
           inherit user;
           group = webserver.group;
           settings = {