Enable source-build with Arcade

This enables 'source-build', which makes it easier to build the entire
shipping .NET SDK from source.

This is the first and second step of arcade-powered-source-build:
https://github.com/dotnet/source-build/blob/master/Documentation/planning/arcade-powered-source-build/README.md

See dotnet/sourcelink#692 for a similar PR, that this is based on.

azure-pipelines.yml

@@ -36,6 +36,7 @@ stages:
       enablePublishTestResults: true
       enablePublishBuildAssets: true
       enablePublishUsingPipelines: $(_PublishUsingPipelines)
+      enableSourceBuild: true
       enableTelemetry: true
       helixRepo: dotnet/symreader
       jobs:
@@ -140,4 +141,4 @@ stages:
     parameters:
       # Symbol validation isn't being very reliable lately. This should be enabled back
       # once this issue is resolved: https://github.com/dotnet/arcade/issues/2871
-      enableSymbolValidation: false
+      enableSymbolValidation: false

eng/SourceBuild.props

@@ -0,0 +1,6 @@
+<Project>
+  <PropertyGroup>
+    <GitHubRepositoryName>symreader</GitHubRepositoryName>
+    <SourceBuildManagedOnly>true</SourceBuildManagedOnly>
+  </PropertyGroup>
+</Project>

eng/SourceBuildPrebuiltBaseline.xml

@@ -0,0 +1,5 @@
+<UsageData>
+  <IgnorePatterns>
+    <UsagePattern IdentityGlob="*/*" />
+  </IgnorePatterns>
+</UsageData>

eng/Version.Details.xml

@@ -6,6 +6,7 @@
     <Dependency Name="Microsoft.DotNet.Arcade.Sdk" Version="6.0.0-beta.21105.12">
       <Uri>https://github.com/dotnet/arcade</Uri>
       <Sha>938b3e8b4edcd96ca0f0cbbae63c87b3f51f7afe</Sha>
+      <SourceBuild RepoName="arcade" ManagedOnly="true"/>
     </Dependency>
   </ToolsetDependencies>
 </Dependencies>