Add support for env var `secrets`

[barjin]

Apify CLI allows passing secret environment variables to the built Actors using apify secrets.

We should support this too here to improve the DX further using this Action.

Blocker for https://github.com/apify/store-website-content-crawler/pull/486

[barjin]

Currently blocked by complicated input parsing in the action.yaml-embedded bash code.

To simplify the implementation of the structured secrets option, we'd need to switch this action to JavaScript implementation.

For an existing structured input implementation, see e.g. Google's get-secretmanager-secrets action.