pip is broken after downgrade on Windows pypy-3.11

Description:
When using the setup-python action with pypy-3.11 on Windows, installing a version of pip older than the version pre-installed by setup-python results in a completely broken pip.

Here is an example workflow run that demonstrates the issue:
https://github.com/bashonly/setup-python-pypy-pip-bug/actions/runs/30870303776

Action version:
reproducible on v7.0.0, v6.3.0, v6.2.0, possibly older versions

Platform:

• Windows

(not reproducible on Ubuntu)

Runner type:

• Hosted

Tools version:
pypy-3.11

Repro steps:

1. Create a GHA workflow with a job that runs on windows-latest
2. Add a step that uses actions/setup-python with python-version: pypy-3.11
3. Add a step that downgrades pip, e.g. python -m pip install "pip==26.1.2"
4. Add a step that attempts to use the downgraded pip, e.g. python -m pip install requests
5. Run the workflow

Here is a reproducer workflow:

https://github.com/bashonly/setup-python-pypy-pip-bug/blob/47691675557d9cc18be1716fbdc2ba690852b9c3/.github/workflows/demo.yml

name: PyPy pip bug MRE
on:
  workflow_dispatch:

permissions:
  contents: read

jobs:
  demonstrate:
    name: Demonstrate PyPy pip issue
    permissions:
      contents: read
    runs-on: windows-latest
    steps:
    - uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6.3.0
      with:
        python-version: pypy-3.11
    - name: Downgrade pip
      run: |
        python -m pip install "pip==26.1.2"
    - name: Use pip
      run: |
        python -m pip install -U requests

Expected behavior:
The downgraded pip should not be broken.

Actual behavior:
The downgraded pip immediately raises an ImportError upon execution.

Analysis:

The root cause seems to be setup-python unconditionally upgrading pip to the latest version using the --ignore-installed pip flag:

https://github.com/actions/setup-python/blob/5fda3b95a4ea91299a34e894583c3862153e4b97/src/install-pypy.ts#L175-L183

The --ignore-installed option tells pip not to uninstall any existing version of the package, but instead to merely install on top of any existing packages. This seems to cause some confusion as to what version is actually installed, as evidenced by the setup-python output from the example workflow run log:

  Installing and updating pip
  C:\hostedtoolcache\windows\PyPy\3.11.15\x64\python.exe -m ensurepip
  Looking in links: c:\Users\RUNNER~1\AppData\Local\Temp\tmpz17a6bza
  Processing c:\users\runner~1\appdata\local\temp\tmpz17a6bza\setuptools-79.0.1-py3-none-any.whl
  Processing c:\users\runner~1\appdata\local\temp\tmpz17a6bza\pip-24.0-py3-none-any.whl
  Installing collected packages: setuptools, pip
    WARNING: The scripts pip3.11.exe and pip3.exe are installed in 'C:\hostedtoolcache\windows\PyPy\3.11.15\x64\Scripts' which is not on PATH.
    Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
  Successfully installed pip-24.0 setuptools-79.0.1
  C:\hostedtoolcache\windows\PyPy\3.11.15\x64\python.exe -m pip install --ignore-installed pip
  Collecting pip
    Downloading pip-26.2-py3-none-any.whl.metadata (4.6 kB)
  Downloading pip-26.2-py3-none-any.whl (1.8 MB)
     ---------------------------------------- 1.8/1.8 MB 38.4 MB/s eta 0:00:00
  Installing collected packages: pip
    WARNING: The scripts pip.exe, pip3.11.exe and pip3.exe are installed in 'C:\hostedtoolcache\windows\PyPy\3.11.15\x64\Scripts' which is not on PATH.
    Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
  Successfully installed pip-24.0

During the upgrade, pip knows it's installing version 26.2, but after installation is complete it reports that version 24.0 is installed. (pip 24.0 is the version that is bootstrapped with ensurepip.)

If we look at the "Use pip" step from example workflow run log, we can see that our attempt to use pip 26.1.2 errors and emits the following traceback:

   File "C:\hostedtoolcache\windows\PyPy\3.11.15\x64\Lib\site-packages\pip\_internal\cli\main_parser.py", line 11, in <module>
    from pip._internal.build_env import get_runnable_pip
  File "C:\hostedtoolcache\windows\PyPy\3.11.15\x64\Lib\site-packages\pip\_internal\build_env\__init__.py", line 8, in <module>
    from pip._internal.build_env.installer import (
  File "C:\hostedtoolcache\windows\PyPy\3.11.15\x64\Lib\site-packages\pip\_internal\build_env\installer.py", line 22, in <module>
    from pip._internal.utils.misc import get_runnable_pip
ImportError: cannot import name 'get_runnable_pip' from 'pip._internal.utils.misc' (C:\hostedtoolcache\windows\PyPy\3.11.15\x64\Lib\site-packages\pip\_internal\utils\misc.py)
Error: Process completed with exit code 1.

_internal\build_env\__init__.py does not exist in pip<26.2. This suggests that the file was left behind from the pip version 26.2 that was installed prior to the downgrade (possibly because the package metadata used to uninstall 26.2 actually belonged to version 24.0). It tries to import get_runnable_pip from pip._internal.utils.misc, but the function wasn't moved into that module until pip version 26.2, so the ImportError is raised.

src/install-pypy.ts の 175-183 行付近にある pip のインストールロジックから始め、次にリンクされている demo.yml ワークフローを windows-latest 上で pypy-3.11 を使って実行します。setup-python のログを downgrade および use-pip ステップと比較します。完了条件は、pip のダウングレードによって Windows 上の pip が使用不能なままにならず、既存の Ubuntu の動作には影響がないことです。