Use GH built-in runner for link check workflow

[Copilot]

Summary

Reduce the job time from 6 minutes to 10 seconds.

Replace pytorch/test-infra reusable workflow + Docker image with direct ubuntu-latest runner for _link_check.yml. The lint scripts only need standard tools (curl, jq, perl, git, grep, sed) already on GH hosted runners. Saves server run hours and cost.

• Switch all three jobs (lint-urls, lint-xrefs, lint-file-size) from linux_job_v2.yml + executorch-ubuntu-22.04-linter Docker image to runs-on: ubuntu-latest with actions/checkout@v4
• Remove id-token: write permission (not needed; calling workflow doesn't grant it)
• Replace fetch-depth: 0 (full clone) with shallow checkout + targeted git fetch --depth=1 of the base SHA
• Replace three-dot (...) with two-dot (..) in git diff across lint_urls.sh, lint_xrefs.sh, lint_file_size.sh — direct base-to-head diff, no merge-base lookup needed

Test plan

CI — the link check jobs run on this PR.

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[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17682

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[Copilot]

Pull request overview

This PR updates the reusable link-check workflow to run directly on GitHub-hosted runners (ubuntu-latest) instead of delegating to the pytorch/test-infra reusable workflow and its Docker image, keeping the existing lint script behavior while reducing runtime and required permissions.

Changes:

• Replaced uses: pytorch/test-infra/.github/workflows/linux_job_v2.yml@main with runs-on: ubuntu-latest + actions/checkout@v4 for each link-check job.
• Set minimal job permissions (contents: read) and removed the previously implied id-token: write need.
• Preserved the existing script invocation logic, including diff-range selection and error messaging.

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