{CI} Fix env-setup action to prevent setuptools downgrade in old PR branches

[Copilot]

Old PR branches that haven't been rebased (e.g., from 2022) carry a stale requirements.txt with setuptools==52.0.0. When azdev setup installs from this file, it silently downgrades setuptools, breaking setuptools.build_meta support — editable installs fail but azdev setup exits 0, leaving the venv without azure.cli. Subsequent steps like scripts/ci/service_name.py then fail with ModuleNotFoundError: No module named 'azure.cli'.

Changes

• .github/actions/env-setup/action.yml: Added a "Use base branch requirements" step between the PR branch checkout and the Python/azdev install. It fetches requirements.txt from the base repo/branch (depth=1, efficient) and checks it out before azdev setup runs, ensuring setuptools is always at a compatible version regardless of how stale the PR branch is.

- name: Use base branch requirements
  shell: bash
  env:
    base_repo: ${{ github.event.pull_request.base.repo.clone_url }}
    base_ref: ${{ github.event.pull_request.base.ref }}
  run: |
    git remote add base "$base_repo" 2>/dev/null || git remote set-url base "$base_repo"
    git fetch base "$base_ref" --depth=1
    git checkout FETCH_HEAD -- requirements.txt

Related command
N/A — CI infrastructure fix only.

Testing Guide
Re-trigger the "GH Check CLI Linter" workflow on PR #21261 (runpy branch). Previously failing with ModuleNotFoundError: No module named 'azure.cli'; should now complete the azdev linter step.

History Notes

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[azure-client-tools-bot-prd]

Hi @copilot,
Since the current milestone time is less than 7 days, this pr will be reviewed in the next milestone.

[azure-pipelines]

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[yonzhan]

/azp run

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