Allow setting environment variables before install

[swissspidy]

Would be nice if it's somehow possible to set PUPPETEER_SKIP_DOWNLOAD=true before the action runs npm ci.

[PixnBits]

FWIW you might be able to do so via jobs.<job_id>.steps[*].env, i.e.:

...
    steps:
      - uses: actions/checkout@v2
      - uses: preactjs/compressed-size-action@v2
        env:
          PUPPETEER_SKIP_DOWNLOAD: true