packages service: fix docker build again (#12774)

CD is hard.

share/spack/qa/run-docker-tests

@@ -36,7 +36,7 @@ this_dir=$(cd $(dirname $0) && pwd)
 SPACK_BIN="${this_dir}/../../../bin/spack"
 
 # packages.spack.io service
-${SPACK_BIN} list --format version_json > packages.json
+${SPACK_BIN} list --format version_json > ${this_dir}/../packages/packages.json
 ./share/spack/packages/build-image.sh
 if [ "$TEST_SUITE" '=' "docker" -a \
      "$TRAVIS_EVENT_TYPE" != "pull_request" ] && ensure_docker_login ; then