[ci] Don't convert nuget to msi in parallel (#6277)

Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=5180358&view=logs&j=5b07041f-3ad2-50e1-dcc0-0b413b70a215&t=aaad43f1-f8e0-5332-447f-d87e37c943fe&l=125
Context: xamarin/yaml-templates#135
Context: xamarin/yaml-templates#134

The `Convert NuGet to MSI` job has been failing consistently since we
updated to a version of dotnet/arcade that introduced some changes to
run in parallel.  I've added a parameter to opt-out of parallelization
until those issues can be fixed.

Additionally, our .NET 6 packs will now be pushed to a public xamarin
`net6-maui-shipping` feed during the `VS Insertion` stage.

build-tools/automation/azure-pipelines.yaml

@@ -1289,6 +1289,7 @@ stages:
         !*Darwin*
       propsArtifactName: nuget-unsigned
       signType: $(MicroBuildSignType)
+      runInParallel: false
       condition: eq(variables['MicroBuildSignType'], 'Real')
 
  # Check - "Xamarin.Android (.NET 6 Prepare Release Push NuGets)"
@@ -1362,6 +1363,9 @@ stages:
     symbolArtifactName: nuget-signed
     symbolArtifactPatterns: |
       !*Darwin*
+    pushToShippingFeed: true
+    nupkgArtifactName: nuget-signed
+    msiNupkgArtifactName: vs-msi-nugets
     condition: eq(variables['MicroBuildSignType'], 'Real')
 
 - stage: finalize_installers