fix(ci): Update artifact download action to use direct repository link

Replaced `actions/download-artifact@v4` with a direct link to
`gitea-download-artifact` repository. This ensures the workflow uses
the latest version of the custom action and improves maintainability.

.gitea/workflows/build-kernel.yml

@@ -93,7 +93,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Download all build output
-        uses: actions/download-artifact@v4
+        uses: https://github.com/christopherHX/gitea-download-artifact@v4
         with:
           path: dist
           # Merge every artifact back into the shared modules/ folder.