[actions] Alternate actions/checkout paths suppress untrusted-checkout alerts

從 actions/ql/lib/codeql/actions/security/UntrustedCheckoutQuery.qll 中的 ActionsMutableRefCheckout.getPath() 和 ActionsSHACheckout.getPath() 開始，然後將它們與 PoisonableSteps.qll 中的 LocalScriptExecutionRunStep.getPath() 進行比較。使用連結的 reproducer 驗證目前的不一致，為列出的路徑形式新增回歸涵蓋，並確認 Critical 和 High 查詢都會回報替代路徑的案例，而 immutable checkout 仍不會被回報。

Description of the issue

actions/untrusted-checkout/critical does not report a privileged workflow that checks out an untrusted PR into a non-default path: and then executes a script or local action from that checkout. The actions/untrusted-checkout/high fallback is also suppressed, so neither query reports the workflow.

Minimal reproducer

on: pull_request_target

permissions:
  contents: write

jobs:
  execute:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
        with:
          repository: ${{ github.event.pull_request.head.repo.full_name }}
          ref: ${{ github.event.pull_request.head.sha }}
          path: candidate
      - run: bash candidate/proof.sh

actions/checkout places path: candidate under $GITHUB_WORKSPACE, so this executes the same checked-out script relationship as a default checkout followed by bash proof.sh.

Reproduction

The complete reproducer is in public fork PR #1.

• Latest-release fork CI: CodeQL Action 4.37.1, CLI 2.26.1, codeql/actions-queries 0.6.31
• Current-main CI: queries compiled directly from github/codeql@14450f5bf38ea9a3ce2e0e45dcf51c0bbdd01af5

Both successful runs scanned all 11 workflow files and produced the same result matrix:

The CI also executed harmless script and composite-action canaries through the tested paths before analysis.

Expected result

The alternate-path cases should be reported by actions/untrusted-checkout/critical, like their default-path controls. A path-representation mismatch should not suppress both the critical query and its high fallback.

Source-level cause

At current main:

• ActionsMutableRefCheckout.getPath() and ActionsSHACheckout.getPath() return explicit checkout paths verbatim.
• LocalScriptExecutionRunStep.getPath() and local-action paths are normalized into GITHUB_WORKSPACE/... form.
• The critical query compares those representations with isSubpath, while the high query suppresses its fallback because the following step is still a PoisonableStep.

Normalizing explicit actions/checkout paths at the checkout model, with regression tests for bare, dot-relative, trailing-slash, and local-action forms, appears to be the smallest fix.