[ci] Fix maestro publishing for stable packages (#9118)

We've seen the build promotion pipeline fail when trying to publish
stable package versions:

    error : Package 'Microsoft.iOS.Ref.net8.0_17.5' has stable version '17.5.8001' but is targeted at a non-isolated feed 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json'

This is because we were not declaring these packages as stable when
building the build asset registry manifest.

Fix this by passing the `$(IsStableBuild)` property to the build asset
manifest creation task. This property needs to be updated manually when
switching to stable package versioning (see commit 4ea5dbb).

When `$(IsStableBuild)` is set to true, packages will be pushed to an
isolated feed during publishing, such as:

      Package [email protected] (Shipping) should go to https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-android-b8317b6f/nuget/v3/index.json (Isolated, Public)

Author: pjcollins

Directory.Build.props

@@ -39,6 +39,8 @@
     <AndroidPackVersionMajorMinor>$(AndroidPackVersion.Substring(0,$(AndroidPackVersionPatchIndex)))</AndroidPackVersionMajorMinor>
     <AndroidPackVersionPatch>$(AndroidPackVersion.Substring($([MSBuild]::Add($(AndroidPackVersionPatchIndex), 1))))</AndroidPackVersionPatch>
     <AndroidPackVersionSuffix>rtm</AndroidPackVersionSuffix>
+    <IsStableBuild>false</IsStableBuild>
+    <IsStableBuild Condition=" '$(AndroidPackVersionSuffix)' == 'rtm' ">true</IsStableBuild>
   </PropertyGroup>
 
   <!-- Common <PackageReference/> versions -->

build-tools/create-packs/Directory.Build.targets

@@ -194,6 +194,7 @@
 
     <PushToAzureDevOpsArtifacts
         ItemsToPush="@(ItemsToPush)"
+        IsStableBuild="$(IsStableBuild)"
         ManifestBuildData="@(ManifestBuildData)"
         ManifestRepoUri="$(BUILD_REPOSITORY_NAME)"
         ManifestBranch="$(BUILD_SOURCEBRANCH)"