fix(ci): revert runner array syntax unsupported by Gitea

Gitea 1.25 does not support array values in matrix includes for
runs-on, causing the dispatch API to return 500. Revert to a single
runner label.

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

.gitea/workflows/build-release.yml

@@ -20,9 +20,7 @@ jobs:
         include:
           - name: cuda13
             fedora_version: "43"
-            runner:
-              - cuda-13.0
-              - fedora-workstation-43
+            runner: cuda-13.0
             cuda_home: /usr/local/cuda-13.0
             cargo_features: "cuda cudnn flash-attn nccl"
             compute_caps: "120"