[3.14] gh-140702: Add test skip for Unix Datagram tests on iOS when on Github Actions (GH-140740) (#140742)

Exposes the GITHUB_ACTIONS environment variable to iOS simulator test runs, and
uses this variable to skip a Unix Datagram socketserver test that is unreliable
in the iOS GitHub Actions environment.
(cherry picked from commit 9f8d005d2961777aa533ec330f96b50324a3446f)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>