Set ImageOS environment variable in runner images

[strideynet]

Is your feature request related to a problem? Please describe.

Several off the shelf actions expect the ImageOS environment variable to be populated (e.g ruby/setup):

Run ruby/setup-ruby@v1
  with:
    ruby-version: 2.6
    bundler: default
    bundler-cache: false
    working-directory: .
    cache-version: 0
Error: The environment variable ImageOS must be set

Describe the solution you'd like

We should set a closest approximation of the ImageOS environment variable in the docker images (e.g ubuntu20)

Describe alternatives you've considered

You can set this as part of the Runner spec.

[callum-tait-pbx]

https://github.com/actions/virtual-environments/blob/b7f276c003aea42575b52247bdb2183e355fca2f/images/linux/scripts/installers/configure-environment.sh

We should probably set it the same way GitHub does for their ubuntu instances i.e. whatever value the below produces:

echo ImageOS=$IMAGE_OS | tee -a /etc/environment

EDIT

I guess the values should be these:

https://github.com/actions/virtual-environments/blob/966395f4bd2afb5c93752996e8738f66fb88dafd/images/linux/ubuntu1804.json#L24
https://github.com/actions/virtual-environments/blob/966395f4bd2afb5c93752996e8738f66fb88dafd/images/linux/ubuntu2004.json#L24

[consideRatio]

This PR is fixed by #730 according to the PR

[toast-gear]

No issues reported since release, closing as resolved.