[wasm-mt] Additional build fixes (#75297)

Foward port changes from `release/7.0` #75171
that were not included in `main` #75162

- when building the InteropServices.JavaScript library, enable
threading if MonoWasmBuildVariant is set appropriately.  One
consequence is that the runtime will (correctly) install the browser
synchronization context on the main thread.

- for workload build testing, unset MonoWasmBuildVariant when creating
the non-threaded runtime

Author: lambdageek

eng/testing/workloads-testing.targets

@@ -171,7 +171,7 @@
       <!-- add for non-threaded runtime also -->
       <_NuGetsToBuild Include="$(LibrariesShippingPackagesDir)Microsoft.NETCore.App.Runtime.Mono.browser-wasm.$(_PackageVersion).nupkg"
                       Project="$(InstallerProjectRoot)pkg/sfx/Microsoft.NETCore.App/Microsoft.NETCore.App.Runtime.sfxproj"
-                      Properties="@(_DefaultPropsForNuGetBuild, ';')"
+                      Properties="@(_DefaultPropsForNuGetBuild, ';');MonoWasmBuildVariant="
                       Dependencies="$(_DefaultRuntimePackNuGetPath)"
                       Descriptor="single threaded runtime pack"
                       Condition="'$(_DefaultBuildVariant)' != '.'" />

src/libraries/System.Runtime.InteropServices.JavaScript/src/System.Runtime.InteropServices.JavaScript.csproj

@@ -6,7 +6,7 @@
   </PropertyGroup>
 
    <PropertyGroup>
-     <FeatureWasmThreads Condition="'$(TargetOS)' == 'browser' and '$(WasmEnableThreads)' == 'true'">true</FeatureWasmThreads>
+     <FeatureWasmThreads Condition="'$(TargetOS)' == 'browser' and ('$(WasmEnableThreads)' == 'true' or '$(MonoWasmBuildVariant)' == 'multithread')">true</FeatureWasmThreads>
    </PropertyGroup>
 
    <PropertyGroup>