Set GIT_DEFAULT_HASH for SHA-256 repo

The repository is initialized with the SHA-256 object format, but
actions/checkout runs `git init` in the workspace before fetching.
Without GIT_DEFAULT_HASH=sha256 that init defaults to SHA-1 and fetch
fails with `mismatched algorithms: client sha1; server sha256`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -35,6 +35,10 @@ on:
 jobs:
   build-release:
     runs-on: ubuntu-latest
+    env:
+      # 仓库使用 SHA-256 对象格式，actions/checkout 内部 `git init` 需在此模式下运行，否则 fetch 会报
+      # `mismatched algorithms: client sha1; server sha256`。
+      GIT_DEFAULT_HASH: sha256
     steps:
       - name: Checkout (full history)
         uses: actions/checkout@v4