nixos/nginx: move configuration testing script into reload command

nginx -t not only verifies configuration, but also creates (and chowns)
files. When the `nginx-config-reload` service is used, this can cause
directories to be chowned to `root`, causing nginx to fail.

This moves the nginx -t command into a second ExecReload command, which
runs as nginx's user. While fixing above issue, this will also cause the
configuration to be verified when running `systemctl reload nginx`, not
only when restarting the dummy `nginx-config-reload` unit. The latter is
mostly a workaround for missing features in our activation script
anyways.