Improve Ubuntu version detection (and prepare for imminent switch to 20.04 by default)

[cptpcrd]

This PR makes the getRunner() function check /etc/os-release to determine the Ubuntu version, rather than hardcoding a kernel version check.

This is also important because the current implementation only checks for Ubuntu 16.04/18.04. Ubuntu 20.04 has been available for use in GitHub Actions for some time, and ubuntu-latest will soon default to ubuntu-20.04 (see actions/virtual-environments#1816).

(Edit: I don't write much Node, and I've never written TypeScript, so I'm happy to make changes to this.)