[browser][mt] Fix package microsoft.netcore.app.runtime.mono.multithread.browser-wasm is not found (#98083)

* Invert the logic: prepare MT nuget for ST and ST nuget for MT.

* We have only 2 runtimes now.

* Added clarification

Author: ilonatommy

eng/testing/workloads-browser.targets

@@ -41,15 +41,16 @@
     </ItemGroup>
   </Target>
 
-  <!-- For local builds, only one of the 3 required runtime packs might be available. In that case,
+  <!-- For local builds, only one of the 2 required runtime packs might be available. In that case,
        build the other nugets with the *same runtime* but different names.
   -->
   <Target Name="_GetRuntimePackNuGetsToBuild" Condition="'$(WasmSkipMissingRuntimePackBuild)' != 'true'" Returns="@(_NuGetsToBuild)">
     <Error Condition="'$(RIDForWorkload)' == ''" Text="$(RIDForWorkload) is unset" />
     <PropertyGroup>
-      <_BuildVariant Condition="'$(WasmEnableThreads)' == 'true'">multithread</_BuildVariant>
-      <_Descriptor Condition="'$(WasmEnableThreads)' == 'true'">runtime pack for $(_BuildVariant)</_Descriptor>
-      <_Descriptor Condition="'$(WasmEnableThreads)' != 'true'">single threaded runtime pack</_Descriptor>
+      <_IsMTNugetMissing Condition="'$(WasmEnableThreads)' != 'true'">true</_IsMTNugetMissing>
+      <_BuildVariant Condition="'$(_IsMTNugetMissing)' == 'true'">multithread</_BuildVariant>
+      <_Descriptor Condition="'$(_IsMTNugetMissing)' == 'true'">runtime pack for $(_BuildVariant)</_Descriptor>
+      <_Descriptor Condition="'$(_IsMTNugetMissing)' != 'true'">single threaded runtime pack</_Descriptor>
       <_DefaultRuntimePackNuGetPath>$([System.String]::Join('.',
         $(LibrariesShippingPackagesDir)Microsoft.NETCore.App.Runtime.Mono,
         $(_BuildVariant),
@@ -67,10 +68,12 @@
            Text="Expected to find either one or two in $(LibrariesShippingPackagesDir): @(_RuntimePackNugetAvailable->'%(FileName)%(Extension)')" />
 
     <ItemGroup>
+      <!-- We need nugets for all wasm runtime flavors. The one corresponding the current 
+      property values is already built, the others need to be added to _NuGetsToBuild -->
       <_NuGetsToBuild Include="$(_DefaultRuntimePackNuGetPath)"
                       Project="$(InstallerProjectRoot)pkg/sfx/Microsoft.NETCore.App/Microsoft.NETCore.App.Runtime.sfxproj"
                       Dependencies="$(_DefaultRuntimePackNuGetPath)"
-                      Properties="@(_DefaultPropsForNuGetBuild, ';');WasmEnableThreads=$(WasmEnableThreads)"
+                      Properties="@(_DefaultPropsForNuGetBuild, ';');WasmEnableThreads=$(_IsMTNugetMissing)"
                       Descriptor="$(_Descriptor)"/>
     </ItemGroup>