Support multi-line commands

[mountainash]

As per https://community.fly.io/t/flyctl-deploy-build-secret-with-multiple-variable-not-wotking/7755/4 it seems the Action is stripping out the carriage return... but not the backslash, resulting in errors.

Expectation

A command like the following (and longer) are much easier to read over multilines. It would be a nice feature if this GH Action supported it.

        run: flyctl deploy ./cms/ \
          --build-secret BASE_PATH="/" \
          --build-secret ENV_MODE="${{ vars.ENV_MODE }}" \
          --build-secret SITE_URL="${{ vars.SITE_URL }}" \
          --build-secret AWS_REGION="${{ vars.AWS_REGION }}" \
          --build-secret AWS_ACCESS_KEY_ID="${{ vars.AWS_ACCESS_KEY_ID }}"

[mountainash]

Thinking about it, I don't think it's an issue of this Action.