Fix _WasmBuildAppCore target not found for CoreCLR browser-wasm samples

Set RuntimeFlavor from Nested_RuntimeFlavor in Directory.Build.props
before the WasmApp.InTree.props import, so that BrowserWasmApp.props
(and WasmApp.Common.props) is not imported for CoreCLR builds.

Previously RuntimeFlavor was only overridden in Directory.Build.targets,
which evaluates after props. Subsets.props defaults RuntimeFlavor to Mono
for browser-wasm, causing the Mono-specific _WasmBuildAppCore target to
be added to WasmNestedPublishAppDependsOn even for CoreCLR builds.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: maraf

src/mono/sample/wasm/Directory.Build.props

@@ -46,6 +46,14 @@
     </ItemGroup>
   </Target>
 
+  <!-- RuntimeFlavor must be set before WasmApp.InTree.props so it imports the correct
+       flavor-specific props (BrowserWasmApp.props for Mono, nothing for CoreCLR).
+       Without this, Subsets.props defaults RuntimeFlavor to Mono for browser-wasm,
+       causing WasmApp.Common.props (Mono) to be imported even for CoreCLR builds. -->
+  <PropertyGroup>
+    <RuntimeFlavor Condition="'$(Nested_RuntimeFlavor)' != ''">$(Nested_RuntimeFlavor)</RuntimeFlavor>
+  </PropertyGroup>
+
   <!-- Import late, so properties like $(ArtifactsBinDir) are set -->
   <Import Project="$(BrowserProjectRoot)build\WasmApp.InTree.props" />
 </Project>