Use CI

msbuild/props/SixLabors.Global.props

@@ -47,7 +47,7 @@
 
   <!-- Ensure deterministic builds work when triggered against individual projects. -->
   <!-- https://github.com/dotnet/roslyn/issues/37379#issuecomment-513371985 -->
-  <ItemGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">
+  <ItemGroup Condition="'$(CI)' == 'true'">
     <SourceRoot Include="$(MSBuildThisFileDirectory)/"/>
   </ItemGroup>
 

msbuild/props/SixLabors.Src.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
 
   <!-- Add deterministic builds in CI .-->
-  <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">
+  <PropertyGroup Condition="'$(CI)' == 'true'">
     <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
   </PropertyGroup>