Extract SDK version from global.json in Versions.props (#6596)

Instead of syncing the version in two places,
treat global.json as canonical and extract the SDK
version value from it in the build.

eng/Versions.props

@@ -31,7 +31,7 @@
   <PropertyGroup>
     <!-- DotNetCliVersion MUST match the dotnet version in global.json.
          Otherwise, this version of dotnet will not be installed and the build will error out. -->
-    <DotNetCliVersion>6.0.100-preview.3.21202.5</DotNetCliVersion>
+    <DotNetCliVersion>$([System.Text.RegularExpressions.Regex]::Match($([System.IO.File]::ReadAllText('$(MSBuildThisFileDirectory)..\global.json')), '"dotnet": "([^"]*)"').Groups.get_Item(1))</DotNetCliVersion>
     <MicrosoftExtensionsDependencyModelVersion>6.0.0-preview.2.21154.6</MicrosoftExtensionsDependencyModelVersion>
     <MicrosoftNetCompilersToolsetVersion>4.0.0-2.21313.1</MicrosoftNetCompilersToolsetVersion>
     <NuGetBuildTasksVersion>6.0.0-preview.1.107</NuGetBuildTasksVersion>