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

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.

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>