Update BuildTools to 2.1.0-preview2-02621-01, fix up build

[dagood]

• Ported Building non-Windows builds using the CLI's MSBuild corefx#26686 "Building non-Windows builds using the CLI's MSBuild"
• Set <RunMatchingRefApiCompat>false</RunMatchingRefApiCompat> globally. This came from Add target reverse APICompat check for libraries buildtools#1949. I think it should be turned off and baselined (like Enable reverse APICompat for 1:1 libraries corefx#27881) but I'm not familiar with this infra, or if it makes sense to do that in a release/2.0.0 branch.

This was driven by attempting to build on Fedora 26, but I also successfully built on Windows.

Doing this for dotnet/source-build#371 (comment)

[weshaggard]

For the RunMatchingRefApiCompat issue you should just need f71d878. Porting that should make the branches more consistent.

[dagood]

Ah, I didn't think to check the master branch here. I'll port that instead, thanks.

[dagood]

CI's green, and it works in the context of a known-good RHEL source-build on my dev machine. Ok to merge @weshaggard?

[weshaggard]

@dagood are the other changes inline with what is in master? If so go ahead and merge if not I would like to make sure they match master as well.

[dagood]

There were newline differences at the end of the version txt files, so I did another cherry-pick and my only change is using the newer BuildTools. (Also nice to have the cherry-pick traceability.)

I had conflicts cherry-picking the MSBuild change: there were divergent BuildTools updates (resolved in favor of master), and release/2.0.0 doesn't have a dependencies.props file at all so I didn't take that change.

[ericstj]

Why didn't you bring over a buildtools version from the release/2.0.0 branch? This creates unnecessary risk for servicing releases.

[weshaggard]

The primary reason is we use this release/2.0.0 branch in source-build and so it needs to work with the latest BuildTools in order to buildable there.

[dagood]

The alternative I had in mind at the time was to make a release/2.1 branch here. I don't think it's feasible to port everything we need to BuildTools release/2.0.0 since all repos would need to uptake it (and there's a decent number of breaks).