[ci] Remove `-m:1` from Windows "Prepare Solution" step (#7029)

macOS use of `make prepare-external-git-dependencies jenkins` doesn't
involve use of `dotnet build -m:1`, so Windows shouldn't require the
use of `dotnet build -m:1` either.

Update `azure-pipelines.yaml` so that Windows doesn't pass `-m:1` to
the [`DotNetCoreCLI@2`][0] task.

This change doesn't really save any time, but it will help ensure we
don't add something that requires it in the future.

[0]: https://docs.microsoft.com/en-us/azure/devops/pipelines/tasks/build/dotnet-core-cli?view=azure-devops

build-tools/automation/azure-pipelines.yaml

@@ -201,7 +201,7 @@ stages:
       displayName: Prepare Solution
       inputs:
         projects: Xamarin.Android.sln
-        arguments: '-c $(XA.Build.Configuration) -t:Prepare --no-restore -m:1 -p:AutoProvision=true -bl:$(System.DefaultWorkingDirectory)\bin\Build$(XA.Build.Configuration)\dotnet-build-prepare.binlog'
+        arguments: '-c $(XA.Build.Configuration) -t:Prepare --no-restore -p:AutoProvision=true -bl:$(System.DefaultWorkingDirectory)\bin\Build$(XA.Build.Configuration)\dotnet-build-prepare.binlog'
 
     # Build, pack .nupkgs, and extract workload packs to dotnet preview test directory
     - template: yaml-templates\run-dotnet-preview.yaml