Update Directory.Build.props

Author: pwelter34

src/Directory.Build.props

@@ -23,6 +23,10 @@
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">
+    <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
+  </PropertyGroup>
+
   <PropertyGroup>
     <LangVersion>latest</LangVersion>
     <ImplicitUsings>enable</ImplicitUsings>