Optimize for the minimal flavor
As of git-for-windows/git-sdk-64#87, the
minimal flavor is published as an asset of a rolling GitHub Release.
This strikes a much better balance between generating and using the
artifact; Now, it is generated exactly once when an update is available,
and none of the users have to regenerate it.
The consequence is that no longer does setup-git-for-windows-sdk take
over a minute (uncached) to initialize the minimal Git for Windows SDK
(and only subsequent, cached calls would get to the desirable 2-5 second
range). Instead, the minimal Git for Windows SDK now consistently takes
2-5 seconds to initialize, no caching needed.