ci: pin third-party actions to commit SHAs (#5869)

* ci: pin third-party actions to commit SHAs

Pin third-party GitHub Actions in workflow files to immutable 40-char
commit SHAs per the org security policy. Mutable refs like @v4 can be
silently re-pointed by a compromised upstream; SHAs cannot. Trailing
version comments let Dependabot/Renovate continue to manage updates.

Related to [COR-51](https://linear.app/crewai/issue/COR-51).

* ci: disable persist-credentials in pip-audit checkout

Address CodeRabbit feedback on PR #5869: the pip-audit workflow is
read-only and never needs an authenticated git context, so opt out of
persisting the GITHUB_TOKEN in the local git config per the
actions/checkout security guidance.