ci: install tox-gh-actions so each matrix job runs only its own env

[jconstance-amplify]

Summary

• tox.ini has always had a [gh-actions] block mapping each Python version to its own pyNN-unit env, but the tox-gh-actions plugin that activates it was never installed in pr_check.yml — so plain tox tried to run every env in envlist on every matrix job, regardless of which single interpreter actions/setup-python had installed for that job.
• This was silently masked as long as tox treated a missing interpreter as a SKIP. The workflow's unpinned tox>=4.0.9,<5 install recently resolved 4.56.4 (up from 4.53.0 on the last green run), and that version bump flips a missing interpreter into a hard FAIL — breaking every matrix job except whichever Python version(s) happen to already be on the runner's PATH. Confirmed via git show <ruff-migration-commit> --stat: that PR's diff touches neither tox.ini nor pr_check.yml, so this is a pre-existing, unrelated regression, not something introduced by the ruff migration.
• Fix: install tox-gh-actions alongside tox in the workflow so the existing [gh-actions] mapping actually takes effect, scoping each job to just its own env.

Verification

• Reproduced locally: built a throwaway venv with tox==4.56.4 (matching CI) and no tox-gh-actions — tox failed on every env whose interpreter wasn't present locally (py38, py310), exactly matching the CI failure signature (could not find python interpreter matching any of the specs).
• Installed tox-gh-actions into that venv and reran under GITHUB_ACTIONS=true: with a Python 3.13 interpreter, tox now runs only py313-unit and passes; repeated with a Python 3.11 venv, tox runs only py311-unit and passes. This matches exactly what each CI matrix job needs.

Test plan

• Reproduced the CI failure locally with pinned tox==4.56.4.
• Verified the fix scopes tox to a single env under both Python 3.11 and 3.13, matching the CI matrix's per-job behavior.
• CI green on this PR (all 6 matrix jobs: 3.8–3.13).

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