ci: 自动安装 .NET 8 SDK（runner 镜像无 dotnet）

docker.gitea.com/runner-images 未预装 dotnet，加 dotnet-install.sh 步骤。

.gitea/workflows/ci.yml

@@ -18,7 +18,15 @@ jobs:
           rm -rf /tmp/src
 
       - name: 确认 dotnet SDK
-        run: dotnet --version
+        run: |
+          if ! command -v dotnet >/dev/null 2>&1; then
+            echo "dotnet 不存在，安装 .NET 8 SDK..."
+            curl -sSL https://dot.net/v1/dotnet-install.sh -o /tmp/dotnet-install.sh
+            chmod +x /tmp/dotnet-install.sh
+            /tmp/dotnet-install.sh --channel 8.0 --install-dir /usr/share/dotnet
+            ln -s /usr/share/dotnet/dotnet /usr/local/bin/dotnet
+          fi
+          dotnet --version
 
       - name: Restore
         run: dotnet restore BasePlatformDotnet/yh-platform-net.sln
@@ -47,7 +55,15 @@ jobs:
           rm -rf /tmp/src
 
       - name: 确认 dotnet SDK
-        run: dotnet --version
+        run: |
+          if ! command -v dotnet >/dev/null 2>&1; then
+            echo "dotnet 不存在，安装 .NET 8 SDK..."
+            curl -sSL https://dot.net/v1/dotnet-install.sh -o /tmp/dotnet-install.sh
+            chmod +x /tmp/dotnet-install.sh
+            /tmp/dotnet-install.sh --channel 8.0 --install-dir /usr/share/dotnet
+            ln -s /usr/share/dotnet/dotnet /usr/local/bin/dotnet
+          fi
+          dotnet --version
 
       - name: Pack
         run: dotnet pack BasePlatformDotnet/yh-platform-net.sln -c Release -o BasePlatformDotnet/artifacts