Add "restore_tools" that gets a new NuGet version for a workaround

[dagood]

To work around NuGet/Home#3190 the recommended way, core-setup needs a new version of NuGet to use --disable-parallel to limit HTTP parallelism during build_projects restore.

There is a chance that restoring restore_projects will time out just like build_projects does, but there are fewer dependencies so it's less likely.

[dnfclas]

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[dagood]

Errors here are due to case sensitivity incompat between restore in the NuGet version I pull in and build in CLI stage_0. Since the case insensitivity change was merged before the change that adds the workaround into NuGet, there's no compatible NuGet version. Barring another layer of workarounds, this isn't going to work with what's out there.

There is a CLI PR out to support the new casing: dotnet/cli#2722. That would make this PR work, but since that PR also upgrades NuGet, it makes this workaround project redundant. Closing.