`codeql-action/init` Makes Metal Toolchain Unavailable on macOS 26 Runners

从 issue 中的复现流程开始，在 macOS 26 runner 上比较 github/codeql-action/init@v4 前后的 xcrun metal -v。阅读 init action 的入口点，并检查初始化如何影响 Xcode 的选择或 Metal toolchain 的可见性。当所选 Xcode 的 Metal toolchain 仍然可用，并且显示的检查和使用 Metal 编译的构建都成功时，即表示完成。

Description

Swift/iOS builds fail after github/codeql-action/init with Xcode 26 on macOS 26 GitHub-hosted runners when the build needs the Metal toolchain.

The failing build error is:

error: error: cannot execute tool 'metal' due to missing Metal Toolchain; use: xcodebuild -downloadComponent MetalToolchain
Command CompileMetalFile failed with a nonzero exit code

This was originally discussed in actions/runner-images:

• Runner-images issue: https://github.com/actions/runner-images/issues/13014
• Maintainer repro: xcrun metal -v works after Xcode selection, then fails after codeql-action/init: https://github.com/actions/runner-images/issues/13014#issuecomment-4259404015
• See comment by @v-sergei-pyshnoi , pointing to an issue here: https://github.com/actions/runner-images/issues/13014#issuecomment-4303584735

Reproduction outline

jobs:
  codeql-metal-repro:
    runs-on: macos-26
    permissions:
      contents: read
      security-events: write
    steps:
      - uses: actions/checkout@v4
      - name: Select Xcode
        uses: maxim-lobanov/setup-xcode@v1
        with:
          xcode-version: latest-stable
      - name: Check Metal before CodeQL init
        run: |
          xcodebuild -version
          xcrun --find metal
          xcrun metal -v
      - name: Initialize CodeQL
        uses: github/codeql-action/init@v4
        with:
          languages: swift
          build-mode: manual
      - name: Check Metal after CodeQL init
        run: |
          xcrun --find metal
          xcrun metal -v
      - name: Build
        run: |
          # Run the normal xcodebuild command for an iOS app target that compiles Metal files.
          xcodebuild ...

Expected: xcrun metal -v and the xcodebuild invocation continue to find the selected Xcode’s Metal toolchain after CodeQL initialization.

Actual: after codeql-action/init, xcrun metal -v / CompileMetalFile can fail with “missing Metal Toolchain”, even though the runner image appears to have the toolchain available before CodeQL initialization.

Note: Xcode 26 changed Metal Toolchain distribution/visibility, and MacPorts documented extra steps sometimes needed to make the toolchain visible to all users:
https://trac.macports.org/wiki/TahoeProblems#MetaltoolchainisnolongerbundledinXcode