Support passing environment variables

[raakasf]

As Mage currently does not support positional args (magefile/mage#170 or magefile/mage#24) we use environment variables with mage. Could you please add passing environment variables with this action?

[crazy-max]

@raakasf You can already do that with GitHub Actions:

      -
        name: Run Mage
        uses: magefile/mage-action@v1
        with:
          version: latest
          args: build
        env:
          MYENV: value

[raakasf]

Yes, just found out here and closing this.