net8/WorkloadManifest.targets: import only for tfm=net8.0, and not tfm>=net8.0 (emscripten-core#510)

Fixes:
`/root/helix/work/workitem/e/dotnet-latest/sdk-manifests/9.0.100-alpha.1/microsoft.net.workload.emscripten.net8/WorkloadManifest.targets(4,5): warning MSB4011: "/root/helix/work/workitem/e/dotnet-latest/packs/Microsoft.NET.Runtime.Emscripten.3.1.34.Node.linux-x64/9.0.0-alpha.1.23469.6/Sdk/Sdk.props" cannot be imported again. It was already imported at "/root/helix/work/workitem/e/dotnet-latest/sdk-manifests/9.0.100-alpha.1/microsoft.net.workload.emscripten.current/WorkloadManifest.targets (23,5)". This is most likely a build authoring error. This subsequent import will be ignored. `

Author: radical

eng/nuget/Microsoft.NET.Workload.Emscripten.net8.Manifest/WorkloadManifest.targets

@@ -1,5 +1,5 @@
 <Project>
-  <ImportGroup Condition="'$(RuntimeIdentifier)' == 'browser-wasm' and '$(UsingBrowserRuntimeWorkload)' == 'true' and '$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals('$(TargetFrameworkVersion)', '8.0'))">
+  <ImportGroup Condition="'$(RuntimeIdentifier)' == 'browser-wasm' and '$(UsingBrowserRuntimeWorkload)' == 'true' and '$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionEquals('$(TargetFrameworkVersion)', '8.0'))">
     <Import Project="Sdk.props" Sdk="Microsoft.NET.Runtime.Emscripten.Python.net8" Condition="!$([MSBuild]::IsOsPlatform('Linux'))" />
     <Import Project="Sdk.props" Sdk="Microsoft.NET.Runtime.Emscripten.Node.net8" />
     <Import Project="Sdk.props" Sdk="Microsoft.NET.Runtime.Emscripten.Sdk.net8" />