Fix AppImage release upload repository

[Zexyen]

Summary

• provide `GH_REPO` to the GitHub CLI release upload step
• allow the release job to identify `tommadness/KH2Randomizer` without a checkout or local Git remote

Root cause

The release job only downloads the built artifact. Since it does not check out the repository, `gh release upload` failed while trying to infer the repository from Git, reporting `fatal: not a git repository`.

Validation

• `git diff --check` passes
• the value comes from the built-in `github.repository` context, so it remains correct when the workflow runs in a fork