Skip package management if running in github-hosted container

GitHub allows running actions in containers [1] on the runners, and when
using a non apt-based Linux distribution, setup-ocaml doesn't work
correctly. This commit skips package management on docker containers,
similar to the behavior on self-hosted runners.

[1]: https://docs.github.com/en/actions/writing-workflows/choosing-where-your-workflow-runs/running-jobs-in-a-container

Author: punchagan