ci: 换 gitea/action-checkout 替代手写 git clone

上一轮改用 git clone 手写 checkout 是因为 actions/checkout@v4 是
Node 实现, alpine golang-1.25 镜像没装 Node. 但后来发现镜像连
bash 也没有 (alpine 极简镜像, 只剩 sh), run: | 块也会崩.

gitea/action-checkout 是 Gitea 官方 Go 实现的 checkout action,
不依赖 Node 也不依赖 bash, 跟 Gitea runner 原生集成. 替换两个
job 的 checkout step.

注意: gitea/action-checkout 只解决 checkout. 后续 run: | 块
(Set GOPROXY / Build / Test / Upload release asset / Notify) 仍
然用 bash, alpine 没 bash 会继续崩. 这是已知遗留, 跑起来再看
是否真崩. 完整修法是给所有 run: | 块加 shell: sh, 但本 commit
先按用户要求只换 checkout, 观察跑结果再决定.

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/build.yml

@@ -22,12 +22,7 @@ jobs:
     runs-on: golang-1.25
     steps:
       - name: Checkout
-        run: |
-          # Manual git checkout — actions/checkout needs Node, which the
-          # alpine golang-1.25 image doesn't have.
-          # Gitea runner doesn't expose a token for clone in the usual
-          # way; for a same-network runner the repo URL is enough.
-          git clone --depth 1 "${{ github.server_url }}/${{ github.repository }}.git" .
+        uses: gitea/action-checkout@v4
 
       - name: Set GOPROXY
         run: |
@@ -69,10 +64,7 @@ jobs:
     runs-on: golang-1.25
     steps:
       - name: Checkout
-        run: |
-          # Manual git checkout — actions/checkout needs Node, which the
-          # alpine golang-1.25 image doesn't have.
-          git clone --depth 1 "${{ github.server_url }}/${{ github.repository }}.git" .
+        uses: gitea/action-checkout@v4
 
       - name: Set GOPROXY
         run: |