ci(Release Notes): add missing environment variables

[AprilSylph]

Description

Adds GITHUB_API_URL and GITHUB_REPOSITORY environment variables to the Release Notes workflow.

Adding the former is more of a completeness thing (it's available on the github context, so why not use it) while the latter should enable this workflow to work properly on forks, instead of always querying AprilSylph/XKit-Rewritten.

Testing steps

See https://github.com/AprilSylph/XKit-Rewritten/actions/runs/23543601713 to verify that the workflow is not broken in the source repository by this change.

To validate that this has the intended effect on forks, you will need to:

1. Copy this branch to your fork
2. Drop one or more src/-affecting commits from your fork's default^[?] branch (or just don't update it, if it's behind upstream), to make it differ
3. Run the modified workflow in your fork
   • Expected result: The commits printed match those present on your fork's default^[?] branch