ci: pin upload-artifact to v3 — v4+ unsupported on forgejo

Forgejo Actions only speaks the GHES-compatible @actions/artifact
protocol; upload-artifact@v4+ insists on the newer API and fails with
`GHESNotSupportedError`. Pin to v3, which uses the old protocol that
Forgejo implements.

Good news: the ISO itself built end-to-end in ~5m on the runner
(DooD + /data symlink resolved the path-mismatch). Only the upload
failed, and this pins it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.forgejo/workflows/build-iso.yml

@@ -34,7 +34,9 @@ jobs:
           sha256sum "$iso"
 
       - name: Upload ISO artifact
-        uses: actions/upload-artifact@v4
+        # v4+ isn't supported on Forgejo yet (uses newer @actions/artifact
+        # protocol that Forgejo's GHES-compatible API doesn't implement).
+        uses: actions/upload-artifact@v3
         with:
           name: furtka-iso
           path: iso/out/*.iso