Update CoreSetup to preview2-25616-02 (master)

[dotnet-bot]

/cc @dotnet/dotnet-cli

[nguerrera]

@Petermarcu @leecow Why is 2.1.0 runtime already at preview2?

[leecow]

@nguerrera I don't know why core-setup is preview2. @dagood ?

[dagood]

Looking at the latest commit on https://github.com/dotnet/core-setup/blob/master/branchinfo.txt, I see dotnet/core-setup#2755:

release/uwp6.0 is going to be using preview1

[steveharter]

@ellismg PTAL (why preview2)

[nguerrera]

#7264 is getting past the managed package version issues by pinning them and starting with just the runtime. Otherwise we get ref 2.1 greater than def 2.0 in stage 0.

However, #7264 is hitting repeatable test regressions. Something is breaking msbuild exec task temporary scripts. I plan to pick this up again next week but if someone wants to investigate sooner, start with debugging that PR, not this.