ci: switch to Tsinghua mirror for apt-get in CI workflow

Use Tsinghua University mirror for faster package downloads in China region

.gitea/workflows/build.yml

@@ -35,7 +35,10 @@ jobs:
           go env GOMODCACHE
 
       - name: Install build tools
-        run: sudo apt-get update && sudo apt-get install -y gcc
+        run: |
+          sudo sed -i 's|http://archive.ubuntu.com|https://mirrors.tuna.tsinghua.edu.cn|g' /etc/apt/sources.list
+          sudo sed -i 's|http://security.ubuntu.com|https://mirrors.tuna.tsinghua.edu.cn|g' /etc/apt/sources.list
+          sudo apt-get update && sudo apt-get install -y gcc
 
       - name: Download dependencies
         run: go mod download