Prevent restoring illink for native-binplace.proj (dotnet#92289)

Fixes dotnet#92194. The
reference to illink from `native-binplace.proj`, built as a
reference of `build-native.proj`, was hitting a nuget bug with
static graph restore. The bug seems to be specific to something
about the project file (maybe the language-specific targets,
since `native-binplace.proj` imports the `Microsoft.NET.Sdk`, but
doesn't have a `csproj` extension).

Fixed by explicitly marking this as not a source project, which
will prevent the import of illink.targets.

src/libraries/Directory.Build.props

@@ -6,6 +6,10 @@
          we only enable it in specific projects. so to avoid duplicating this property in coreclr, we can first scope it to src/libraries.
          This property needs to be declared before the ..\..\Directory.Build.props import. -->
     <SkipArcadeNoWarnCS1591>true</SkipArcadeNoWarnCS1591>
+
+    <!-- Prevent native-binplace.proj from being treated as a source project, to avoid importing illink targets
+         and thereby avoid hitting a static graph restore bug: https://github.com/dotnet/runtime/issues/92194 -->
+    <IsSourceProject Condition="'$(MSBuildProjectFile)' == 'native-binplace.proj'">false</IsSourceProject>
   </PropertyGroup>
 
   <Import Project="..\..\Directory.Build.props" />