make github server url customizable when using github enterprise

[YanyChoi]

• It was impossible to override environment variables GITHUB_SERVER_API and GITHUB_API_URL inside the prepare step.
• This made it impossible to use claude code with github enterprise.
• Make both environment variables inserted when calling the step, and use the default value when not declared.

[ashwin-ant]

Can you share why this is needed? These should be included by default in the GHE workflow environment.

[YanyChoi]

I thought so too, but the environment variables were only available on the job, but it didn't get inserted to the prepare step unless i inserted it through the step configuration's env settings.