[Blazor] Allow customizing the static web assets base manifest paths if defined by the environment (#27102)

(cherry picked from commit bcb1cd2)

Author: javiercn

src/RazorSdk/Targets/Microsoft.NET.Sdk.Razor.StaticWebAssets.targets

@@ -296,7 +296,7 @@ Copyright (c) .NET Foundation. All rights reserved.
 
   <Target Name="ResolveStaticWebAssetsConfiguration">
     <PropertyGroup>
-      <_StaticWebAssetsManifestBase>$(IntermediateOutputPath)</_StaticWebAssetsManifestBase>
+      <_StaticWebAssetsManifestBase Condition="'$(_StaticWebAssetsManifestBase)' == ''">$(IntermediateOutputPath)</_StaticWebAssetsManifestBase>
       <StaticWebAssetBasePath Condition="'$(StaticWebAssetBasePath)' == ''">_content/$(PackageId)</StaticWebAssetBasePath>
       <StaticWebAssetProjectMode Condition="'$(StaticWebAssetProjectMode)' == ''">Default</StaticWebAssetProjectMode>
       <StaticWebAssetBuildManifestPath>$(_StaticWebAssetsManifestBase)staticwebassets.build.json</StaticWebAssetBuildManifestPath>