how to write selectors for mappings?

[kratsg]

when using recipe v1 format, how can we write the following

build:
  number: 0
  skip: win
  script:
    env:
      CXXFLAGS: "-Wall"  # [linux]

into v1 format? The below does not work

build:
  number: 0
  skip: win
  script:
    env:
      - if: unix
        then:
          - CXXFLAGS: "-Wall"

as script.env requires a mapping.

[lucascolley]

build:
  number: 0
  skip: win
  script:
    env:
      CXXFLAGS: ${{ "-Wall" if unix else "" }}

Maybe this works? Definitely not ideal though...