Attempt to find latest MSBuild version when none specified

When no Version is specified in the configuration, this update attempt to find the latest MSBuild version that exists.  Otherwise, it behaves as normal (use the specified version, and fail if it doesn't exist)

Open to any feedback here - unsure if this is the best route and could use more testing, but just wanted to make sure this was on the right track.

One "issue" to note is that if it falls all the way back to `v3.5`, there's an error because `maxcpucount` is not supported (and possibly other flags?)