Improve CI build performance.

Is this measurable? I'm curious to know if it made a remarkable impact on build times (as in going from X to Y minutes where Y < X).

when you opt out from first time experience, the global cache doesn't get populated. it takes ~50s to populate it
http://donovanbrown.com/post/Stop-wasting-time-during-NET-Core-builds

@filipw Thanks and I was precisely looking for that text quoted by @DarqueWarrior on his blog post:

A command is running to initially populate your local package cache, to improve restore speed and enable offline access. This command will take up to a minute to complete and will only happen once.

I didn't find it on previous builds in the build history (AppVeyor/Travis), taking a recent build for b390197 as an example. Then I would have expected that it disappeared in the build for this PR. It could be that the images used AppVeyor and Travis have the cache primed so those build are not considered first-time runs.