fix(build): lock musl outputs to fully static linking in build script and CI workflows

[Suyunmeng]

Description / 描述

在构建脚本中统一 musl 的 ldflags，并强制外部静态链接模式。
在构建脚本中增加产物静态性校验，若出现动态链接则立即失败。
在 Beta Release 工作流中为 musl 矩阵显式设置静态 ldflags。
显式开启 cgo-actions 的 static-link-for-musl。
在工作流中增加 readelf 静态校验步骤，防止动态 musl 产物被打包发布。
保持现有 x-flags 注入逻辑，不影响非 musl 目标行为。

Motivation and Context / 背景

部分 musl 目标在某些构建路径下出现了动态链接产物，导致在无兼容 musl 运行时/加载器环境中无法执行。
本次修改通过参数锁定和校验步骤双重保证静态产物，并在 CI 阶段及时拦截回归

fix #2296

How Has This Been Tested? / 测试

已在脚本与 CI 工作流中加入确定性的静态校验（readelf 解释器段检测）。

全量跨平台产物结果将通过后续 CI 运行进一步确认。

Checklist / 检查清单

• I have read the CONTRIBUTING document.
  我已阅读 CONTRIBUTING 文档。
• I have formatted my code with go fmt or prettier.
  我已使用 go fmt 或 prettier 格式化提交的代码。
• I have added appropriate labels to this PR (or mentioned needed labels in the description if lacking permissions).
  我已为此 PR 添加了适当的标签（如无权限或需要的标签不存在，请在描述中说明，管理员将后续处理）。
• I have requested review from relevant code authors using the "Request review" feature when applicable.
  我已在适当情况下使用"Request review"功能请求相关代码作者进行审查。
• I have updated the repository accordingly (If it’s needed).
  我已相应更新了相关仓库（若适用）。
  • OpenList-Frontend #XXXX
  • OpenList-Docs #XXXX

[Copilot]

Pull request overview

This PR hardens musl build outputs to be fully statically linked by centralizing musl linker flags, enforcing external static link mode, and adding CI + script-level verification to fail fast if any musl artifact ends up dynamically linked.

Changes:

• Centralizes musl static -ldflags in build.sh and applies them across musl build paths, plus adds per-artifact static verification.
• Forces musl builds to prefer static external linking via CGO_LDFLAGS=-static and CI/action configuration (static-link-for-musl: true).
• Adds readelf-based CI verification steps to block publishing dynamic musl binaries.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
build.sh	Adds centralized musl ldflags helper + static-binary assertion; applies static flags across musl build targets.
.github/workflows/build.yml	Forces musl ldflags/static-link mode in PR builds and adds a musl static verification step.
.github/workflows/beta_release.yml	Forces musl ldflags/static-link mode in beta release builds and adds a musl static verification step over produced artifacts.

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