Install matrix-specific pytest after test-requirements so pre-release job actually tests pre-release

[Copilot]

The pytest@pre-release entry in the python-tests matrix was not running against pre-release pytest. build/test-requirements.txt pins pytest<8.1 (required by pytest-black 0.6.0, added in #26009), and it was being installed after the matrix-specific pytest step — silently reverting pre-release/latest pytest back to 8.0.2.

Changes to .github/workflows/pr-check.yml

• Reordered install steps so test-requirements.txt runs before the matrix-specific pytest install, letting the matrix version win instead of being overridden.
• Added --upgrade to the specific-pytest step so the plain pytest entry actually moves past the <8.1 pin to the current stable release (previously also silently held at 8.0.2).
• Renamed the pytest --version step from the duplicated "Install specific pytest version" to "Print pytest version".

The pytest<8.1 pin in test-requirements.txt is retained so pytest-black continues to work; the matrix override now correctly supersedes it for the pre-release and stable jobs.

- name: Install test requirements
  run: python -m pip install --upgrade -r build/test-requirements.txt

# Installed AFTER test-requirements so the pytest<8.1 pin (needed for
# pytest-black) does not clobber the matrix-selected version.
- name: Install specific pytest version (pre-release)
  if: matrix.pytest-version == 'pytest@pre-release'
  run: python -m pip install --upgrade --pre pytest

- name: Install specific pytest version
  if: matrix.pytest-version != 'pytest@pre-release'
  run: python -m pip install --upgrade "${{ matrix.pytest-version }}"

Note: with pre-release pytest now genuinely in effect, the 2496-black-formatter discovery test may surface pytest-black incompatibilities on newer pytest — which is the intended signal of the pre-release job.