update drone.yml

drone.yml

     - mkdir -p dist/etc/nginx/snippets
     - cp -p config/nginx-datasrc-location.conf dist/etc/nginx/snippets/nginx-datasrc-location.conf
     - RELEASE=${DRONE_REPO_NAME}
-    - [ -n "${DRONE_REPO_BRANCH}" ] && RELEASE=${RELEASE}-${DRONE_REPO_BRANCH}
-    - [ -n "${DRONE_TAG}" ] && RELEASE=${RELEASE}-${DRONE_TAG}
+    - \[ -n "${DRONE_REPO_BRANCH}" \] && RELEASE=${RELEASE}-${DRONE_REPO_BRANCH}
+    - \[ -n "${DRONE_TAG}" \] && RELEASE=${RELEASE}-${DRONE_TAG}
     - mv dist $RELEASE
     - ls -la $RELEASE ; find $RELEASE -type f -exec md5sum '{}' \;
     - rm -rf release ; mkdir release