Make CI builds deterministic

Author: Carl-Hugo

src/Directory.Build.props

@@ -15,6 +15,9 @@
         <Nullable>enable</Nullable>
         <ImplicitUsings>enable</ImplicitUsings>
     </PropertyGroup>
+    <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">
+        <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
+    </PropertyGroup>
     <ItemGroup>
         <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All"/>
         <PackageReference Include="Nerdbank.GitVersioning">