fix: correct shellcheck issue

tests/setup.sh

@@ -2,7 +2,7 @@
 set -eo pipefail
 [[ $TRACE ]] && set -x
 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 762E3157
-echo "deb http://nginx.org/packages/ubuntu `lsb_release -cs` nginx" | sudo tee /etc/apt/sources.list.d/nginx.list
+echo "deb http://nginx.org/packages/ubuntu $(lsb_release -cs) nginx" | sudo tee /etc/apt/sources.list.d/nginx.list
 curl -fsSL https://nginx.org/keys/nginx_signing.key | sudo apt-key add -
 
 wget https://raw.githubusercontent.com/dokku/dokku/master/bootstrap.sh