Support file type token variables

We are using file token variables for our project, because we don't want to leak their content when scripts in other jobs are exposing their value in the artifacts.

So it would be nice if running with ISSUE_BOT_API_TOKEN_FILE would be accepted too.

Without this we cannot run the image as per:

2025/10/30 02:33:49 unable to create GitLab client(s): please configure ISSUE_BOT_API_TOKEN environment variable (unable to find environment variable: ISSUE_BOT_API_TOKEN)