ci: use Tsinghua apt/apk mirrors on the runner

Install was pulling bookworm packages from deb.debian.org; the domestic
Gitea runner crawls that for minutes. Rewrite sources to TUNA before
apt-get/apk, with CST_CI_MIRROR to override.

.gitea/workflows/ci.yml

@@ -16,6 +16,7 @@ jobs:
       - name: Install dependencies
         run: |
           SUDO=$(command -v sudo || true)
+          $SUDO bash scripts/ci_pkg_mirror.sh
           $SUDO apt-get update
           $SUDO apt-get install -y --no-install-recommends \
             build-essential clang cmake pkg-config python3 \
@@ -42,6 +43,7 @@ jobs:
       - name: Install dependencies
         run: |
           SUDO=$(command -v sudo || true)
+          $SUDO bash scripts/ci_pkg_mirror.sh
           $SUDO apt-get update
           $SUDO apt-get install -y --no-install-recommends \
             build-essential pkg-config libcurl4-openssl-dev libncurses-dev
@@ -53,7 +55,11 @@ jobs:
     container: alpine:3.20
     steps:
       - name: Install dependencies
-        run: apk add --no-cache build-base pkgconf curl-dev ncurses-dev git nodejs bash python3
+        run: |
+          # Container has no checkout yet; rewrite apk repos in place.
+          sed -i 's#https\?://dl-cdn.alpinelinux.org/alpine#https://mirrors.tuna.tsinghua.edu.cn/alpine#g' \
+            /etc/apk/repositories
+          apk add --no-cache build-base pkgconf curl-dev ncurses-dev git nodejs bash python3
       - uses: actions/checkout@v4
       - name: Build + unit tests
         run: |