cannot use windows for global runner

[ashleygwilliams]

in 0.15.0 we enabled the ability to use a custom runner for the global task. this generally works well because the users who have opted to use this feature are opting for linux runners. the global task workflow is currently hardcoded to only use the shell installer on bash, which i believe would not work on a windows custom runner. thankfully, it's almost certainly true that very few people would select a windows runner for the global task, so this is a bug with little to no impact. that being said, we should square this at some point, and it may in fact simplify things, as it makes the "plan" task less "special"