Bump .NET Core SDK to the latest (2.1.512) (#19257)

* Bump .NET Core SDK to the latest (2.1.512)
- use just-built aspnet/BuildTools

* Clean up build of templating submodule
- response file otherwise contains invalid `/p:RepositoryRoot="...\Templating\\\"`

Author: dougbu

build/RepositoryBuild.targets

@@ -119,8 +119,9 @@
 
     <Message Text="============ Building $(RepositoryToBuild) ============" Importance="High" />
 
+    <!-- Remove final slash in $(BuildRepositoryRoot) to avoid problems due to KoreBuild adding one on Windows. -->
     <Exec
-      Command="./$(_BuildScriptToExecute) -Path $(BuildRepositoryRoot) $(BuildArguments)"
+      Command="./$(_BuildScriptToExecute) -Path $(BuildRepositoryRoot.TrimEnd('\\')) $(BuildArguments)"
       IgnoreStandardErrorWarningFormat="true"
       WorkingDirectory="$(RepositoryRoot)"
       IgnoreExitCode="true"
@@ -163,8 +164,9 @@
 
     <Message Text="============ Testing $(RepositoryToBuild) ============" Importance="High" />
 
+    <!-- Remove final slash in $(BuildRepositoryRoot) to avoid problems due to KoreBuild adding one on Windows. -->
     <Exec Condition="'$(SkipTestsDueToMissingSharedFx)' != 'true' "
-      Command="./$(_BuildScriptToExecute) -Path $(BuildRepositoryRoot) $(BuildArguments)"
+      Command="./$(_BuildScriptToExecute) -Path $(BuildRepositoryRoot.TrimEnd('\\')) $(BuildArguments)"
       IgnoreStandardErrorWarningFormat="true"
       WorkingDirectory="$(RepositoryRoot)"
       IgnoreExitCode="true">

korebuild-lock.txt

@@ -1,2 +1,2 @@
-version:2.1.7-build-20190110.2
-commithash:00aefcfd284db33352f35bfa2c74c65f8580e372
+version:2.1.7-build-20200221.1
+commithash:96fb43f7c4a724d7af26b053240182760ac80a9b