Fix build and test image job by correcting external checkout target in ISO workflow

[Copilot]

The build and test image job was failing in Actions due to an invalid/mis-scoped checkout target in the ISO pipeline. This change updates the workflow to checkout the correct repository source so the image build stage can run end-to-end instead of failing during setup.

• Root cause

  • The ISO workflow attempted to checkout radix-gnu-linux/radix in the build job, which caused setup failure in the failing run (93615082902).

• Workflow fix

  • Updated the checkout step in .github/workflows/build-iso.yml to use the correct repository reference for this pipeline context.
  • Kept the existing checkout path/layout intact to avoid downstream script changes.

• Why this is minimal

  • No build logic or image scripts were modified; only the repository source for the checkout step was corrected.

- uses: actions/checkout@v4
  with:
-   repository: radix-gnu-linux/radix
+   repository: radix-gnu-linux/radix-linux
    path: radix
    persist-credentials: false