ci: 构建容器改用 ubuntu:20.04（glibc 2.31 基线不变）

替代 debian:11（bullseye 已 EOL，镜像进入元数据过期+pool 清理
半死状态）和 debian:12（glibc 2.36 会放弃 Ubuntu 22.04 等系统，
违背 v4.3.1 的 2.31 基线承诺）。

ubuntu:20.04 (focal) glibc 同为 2.31：
- 门禁 GLIBC_MIN=2.31 / deb 声明 libc6 (>= 2.31) / README 承诺全部不变
- focal 仍在 archive.ubuntu.com 正常服务（主仓/security 均 200），
  且 Ubuntu Release 无 Valid-Until 字段，不存在 Debian 的过期检查问题
- 产物兼容性完全等价：Debian 11/12/13、Ubuntu 20.04/22.04/24.04+

Author: lloydzhou

.github/workflows/ci.yml

@@ -95,7 +95,7 @@ jobs:
       - name: Build goagent (linux, Debian 11 container)
         if: matrix.goos == 'linux'
         run: |
-          docker run --rm -v /opt/hostedtoolcache:/opt/hostedtoolcache -v "$PWD":/github/workspace -w /github/workspace debian:12 \
+          docker run --rm -v /opt/hostedtoolcache:/opt/hostedtoolcache -v "$PWD":/github/workspace -w /github/workspace ubuntu:20.04 \
             bash -euxo pipefail -c '
               GO_BIN="$(ls -d /opt/hostedtoolcache/go/*/x64/bin | sort -V | tail -1)/go"
               export PATH="$(dirname "$GO_BIN"):$PATH"
@@ -183,7 +183,7 @@ jobs:
       - name: Build rustagent (linux, Debian 11 container)
         if: matrix.runner == 'ubuntu-latest'
         run: |
-          docker run --rm -v "$PWD":/github/workspace -w /github/workspace debian:12 \
+          docker run --rm -v "$PWD":/github/workspace -w /github/workspace ubuntu:20.04 \
             bash -euxo pipefail -c '
               apt-get update
               apt-get install -y --no-install-recommends curl ca-certificates build-essential pkg-config
@@ -258,7 +258,7 @@ jobs:
       - name: Build webagent (linux, Debian 11 container)
         if: matrix.runner == 'ubuntu-latest'
         run: |
-          docker run --rm -v "$PWD":/github/workspace -w /github/workspace debian:12 \
+          docker run --rm -v "$PWD":/github/workspace -w /github/workspace ubuntu:20.04 \
             bash -euxo pipefail -c '
               apt-get update
               apt-get install -y --no-install-recommends curl ca-certificates build-essential pkg-config
@@ -313,7 +313,7 @@ jobs:
       - name: Build cagent (linux, Debian 11 container)
         if: runner.os == 'Linux'
         run: |
-          docker run --rm -v "$PWD":/github/workspace -w /github/workspace debian:12 \
+          docker run --rm -v "$PWD":/github/workspace -w /github/workspace ubuntu:20.04 \
             bash -euxo pipefail -c '
               apt-get update
               apt-get install -y --no-install-recommends gcc make xxd libc6-dev libcurl4-openssl-dev pkg-config