Backslashes are escape characters. Trying forward slashes explicitly.

src/finalizer/CMakeLists.txt

@@ -12,9 +12,9 @@ set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /SUBSYSTEM:WINDOWS /ENTRY:
 # The WiX SDK is extracted from a NuGet package using an SDK .csproj (finalizer-build) that copies the "lib" and "inc" folders to a stable location.
 # The defines below (values in ${}) are set in the finalizer.nativeproj.
 # See: https://github.com/dotnet/arcade/tree/main/src/Microsoft.DotNet.CMake.Sdk#common-items
-include_directories(${ArtifactsDir}WixSdk\inc)
+include_directories(${ArtifactsDir}WixSdk/inc)
 include_directories(${ArtifactsDir}obj)
-link_directories(${ArtifactsDir}WixSdk\lib\${Platform})
+link_directories(${ArtifactsDir}WixSdk/lib/${Platform})
 
 add_compile_options(/MT)
 

src/finalizer/finalizer.nativeproj

@@ -9,9 +9,20 @@
   <!-- This will be passed through -D and referenced by the CMakeLists.txt to correctly resolve the platform specific libs. -->
   <ItemGroup>
     <CMakeDefines Include="Platform" Value="$(Platform)" />
-    <CMakeDefines Include="ArtifactsDir" Value="$(ArtifactsDir)" />
+    <CMakeDefines Include="ArtifactsDir" Value="$([System.String]::Copy($(ArtifactsDir)).Replace('\','/'))" />
     <!-- Include this item below to create a binlog for the native build. -->
     <!-- <CMakeNativeToolArguments Include="/bl:$(ArtifactsDir)native.binlog" /> -->
   </ItemGroup>
 
+  <Target Name="FinalizerPropertyCheck" AfterTargets="Restore" BeforeTargets="Build">
+    <Message Text="Platform: $(Platform)" Importance="high"/>
+    <Message Text="ArtifactsDir: $(ArtifactsDir)" Importance="high"/>
+
+    <PropertyGroup>
+      <ArtDirFixed>$([System.String]::Copy($(ArtifactsDir)).Replace('\','/'))</ArtDirFixed>
+    </PropertyGroup>
+
+    <Message Text="ArtDirFixed: $(ArtDirFixed)" Importance="high"/>
+  </Target>
+
 </Project>