Use correct bash test condition

Author: jradtilbrook

packer/linux/conf/bin/bk-install-elastic-stack.sh

@@ -101,7 +101,7 @@ if [[ "${BUILDKITE_AGENT_ENABLE_GIT_MIRRORS_EXPERIMENT}" == "true" ]] ; then
 fi
 
 # If the agent token path is set, use that instead of BUILDKITE_AGENT_TOKEN
-if [[ -z "${BUILDKITE_AGENT_TOKEN_PATH}" ]] ; then
+if [[ -n "${BUILDKITE_AGENT_TOKEN_PATH}" ]] ; then
     BUILDKITE_AGENT_TOKEN="$(aws ssm get-parameter --name "${BUILDKITE_AGENT_TOKEN_PATH}" --with-decryption --query Parameter.Value)"
 fi