message: ci: Use CMAKE_BUILD_PARALLEL_LEVEL with 8 by default
  The env var CMAKE_BUILD_PARALLEL_LEVEL sets the parallel level of
  cmake --build.
diff: @@ -118,7 +118,9 @@ commands:
  - ~/.hunter/_Base/Cache
  - run:
  name: "Build <<parameters.configuration_name>> (<<parameters.build_type>>)"
  - command: cmake --build ~/build --target <<parameters.target>> -- -j6
  + command: |
  + export CMAKE_BUILD_PARALLEL_LEVEL=${CMAKE_BUILD_PARALLEL_LEVEL:-8}
  + cmake --build ~/build --target <<parameters.target>>
  test:
  description: "Test"
  