[build] fix generate and publish BAR manifest step (#10685)

This failed with:

    Microsoft.NET.TargetFrameworkInference.targets(188,5): error NETSDK1045:
    The current .NET SDK does not support targeting .NET 11.0.
    Either target .NET 10.0 or lower, or use a version of the .NET SDK that supports .NET 11.0.
    Download the .NET SDK from https://aka.ms/dotnet/download

This step doesn't *need* to use .NET 11, so...

I think we can use `$(DotNetStableTargetFramework)` in this project
to avoid this problem for now.

Author: jonathanpeppers

build-tools/create-packs/Microsoft.Android.Sdk.proj

@@ -11,6 +11,11 @@ core workload SDK packs imported by WorkloadManifest.targets.
   <Sdk Name="Microsoft.DotNet.SharedFramework.Sdk" Version="$(MicrosoftDotNetBuildTasksFeedPackageVersion)" />
   <UsingTask AssemblyFile="$(BootstrapTasksAssembly)" TaskName="Xamarin.Android.Tools.BootstrapTasks.GenerateUnixFilePermissions" />
 
+  <PropertyGroup>
+    <!-- Use stable TFM since CI only has the stable .NET SDK installed -->
+    <TargetFramework>$(DotNetStableTargetFramework)</TargetFramework>
+  </PropertyGroup>
+
   <PropertyGroup>
     <PackageId>Microsoft.Android.Sdk.$(HostOS)</PackageId>
     <OverridePackageId>$(PackageId)</OverridePackageId>