Bump .NET Core SDK to the latest (2.1.512)

[dougbu]

• use just-built aspnet/BuildTools

[dougbu]

/fyi @vivmishra

[dougbu]

@Pilchie any objection to tell-mode for this change? The hope is that moving to the new SDK avoids the errors we're now hitting in 2.1.17 official builds:

[12:21:37]Could not find a version of MSBuild which satisfies the requirements specified 
[12:21:37]in korebuild.json.

[dougbu]

Shoot, the validation build failed on Windows! Ideas please.

2020-02-22T01:56:45.2741613Z   17. Templating
2020-02-22T01:56:53.5890308Z   ============ Building Templating ============
2020-02-22T01:56:55.7754469Z   Using KoreBuild 2.1.7-build-20200221.1
2020-02-22T01:56:55.8814221Z   Adding F:\workspace\_work\1\.dotnet\x64 to PATH
2020-02-22T01:56:55.8860614Z   .NET Core SDK 2.1.512 is already installed. Skipping installation.
2020-02-22T01:56:55.9161625Z   >>> dotnet.exe msbuild -t:restore -v:q F:\workspace\_work\1\.dotnet\buildtools\korebuild\2.1.7-build-20200221.1\scripts/../modules/BundledPackages/BundledPackageRestorer.csproj
2020-02-22T01:56:56.2045428Z   Microsoft (R) Build Engine version 15.9.20+g88f5fadfbe for .NET Core
2020-02-22T01:56:56.2047471Z   Copyright (C) Microsoft Corporation. All rights reserved.
2020-02-22T01:56:56.2048062Z   
2020-02-22T01:56:57.1648629Z   >>> dotnet.exe msbuild @F:\workspace\_work\1\s\src\Templating\artifacts\logs\msbuild.rsp @F:\workspace\_work\1\s\src\Templating\artifacts\logs\msbuild.logger.rsp
2020-02-22T01:56:57.7149654Z   F:\workspace\_work\1\.dotnet\buildtools\korebuild\2.1.7-build-20200221.1\KoreBuild.Common.targets(81,5): error MSB3191: Unable to create directory "F:\workspace\_work\1\s\src\Templating\"\artifacts\". The filename, directory name, or volume label syntax is incorrect : 'F:\workspace\_work\1\s\src\Templating\"\artifacts\' [F:\workspace\_work\1\.dotnet\buildtools\korebuild\2.1.7-build-20200221.1\KoreBuild.proj]
2020-02-22T01:56:57.7153164Z   F:\workspace\_work\1\.dotnet\buildtools\korebuild\2.1.7-build-20200221.1\KoreBuild.Common.targets(81,5): error MSB3191: Unable to create directory "F:\workspace\_work\1\s\src\Templating\"\artifacts\build\". The filename, directory name, or volume label syntax is incorrect : 'F:\workspace\_work\1\s\src\Templating\"\artifacts\build\' [F:\workspace\_work\1\.dotnet\buildtools\korebuild\2.1.7-build-20200221.1\KoreBuild.proj]

Are those errors familiar to anyone?

I can't see where the extra double quote is coming from though it's likely some strange combination of https://github.com/dotnet/aspnetcore/blob/release/2.1/build/RepositoryBuild.targets#L22 (setting $(BuildRepositoryRoot) to the part ending with Templating\), https://github.com/dotnet/aspnetcore/blob/release/2.1/build/RepositoryBuild.targets#L122-L129 (calling .\build.cmd with the quoted path), the script chain from https://github.com/dotnet/aspnetcore/blob/release/2.1/build.cmd into the msbuild.rsp file, and the templates build's attempt to create its artifacts folders.

More importantly, I can't imagine how updating the .NET Core SDK would cause this new problem.