uses: simenandre/setup-variables@v2
id: stack
with:
  key: ${{ env.CI_REF_NAME_SLUG }}
  map: |
    main: prod
    staging: staging
    dev: dev
    '*': dev

There are multiple other Github Actions step that tries to solve similar/same issues.

variable-mapper for instance, has mostly the same functionality as this. The schema design is mostly the same, only difference being that the original uses JSON for mapping.