Support ENV file

[rlespinasse]

Enhance this GitHub Action to support loading environment variables from an ENV file (or more) when using envsubst. This will improve flexibility and security by allowing users to manage their environment variables separately from the workflow file.

Current Behavior

This GitHub Action requires environment variables to be defined directly in the workflow file. This can lead to cluttered workflow files and makes it difficult to manage multiple environments.

Proposed Behavior

The GitHub Action should be able to read environment variables from a specified ENV file (or more than one) and use them with envsubst.

This will allow users to:

• Keep their workflow files clean and focused on the action logic
• Easily manage multiple environment configurations
• Improve security by storing sensitive data outside of the workflow file