nixos/jenkins: ensure postStart follows redirects

Pass the -L flag to curl to make it follow redirects. This fixes an
issue I found when setting up reverse proxy for Jenkins. Without this
fix, the returned HTTP code was stuck at 302, making postStart fail the
service (it expects 200 or 403).

nixos/modules/services/continuous-integration/jenkins/default.nix

···
       '';
 
       postStart = ''
-        until [[ $(${pkgs.curl.bin}/bin/curl -s --head -w '\n%{http_code}' http://${cfg.listenAddress}:${toString cfg.port}${cfg.prefix} | tail -n1) =~ ^(200|403)$ ]]; do
+        until [[ $(${pkgs.curl.bin}/bin/curl -L -s --head -w '\n%{http_code}' http://${cfg.listenAddress}:${toString cfg.port}${cfg.prefix} | tail -n1) =~ ^(200|403)$ ]]; do
           sleep 1
         done
       '';