only download runner if not already installed

[greg-cook]

This change is aimed at allowing initial support of the Cirrus images
These images come preinstalled with the actions-runner so we can skip to the configuration step.

[simonbs]

Looks good. I'm curious to hear how you plan to support Cirrus' runner images 👀

[NikoRoberts]

hi @simonbs 👋
The way we were looking to use the images was essentially use them inside Tartelet as they already show up in the dropdown of Tart VMs.

But when we tried to run Tartelet it just kept rebooting the VM because the mkdir command was failing.
The reason we were looking at this option was because were hoping to have a fleet of internal devices to provide additional parallelism, but maintain a few Cirrus runners as the core devices that can run the pipeline if our internal hardware isn't available

It also means all the configuration of the accounts on the VM is already done

[simonbs]

@NikoRoberts Thanks for providing the details! This sounds like an interesting approach.