Use the dotnet task's restore command. If you're using a single Azure Artifacts feed, just select it from the dropdown in the task (instead of the PowerShell you mentioned). If multiple feeds (doesn't look like it from your question, but just in case), you'll need to check in a NuGet.config that references all of those feeds, then point the task to that config.
You may also need to pass the '--no-restore' flag to your 'dotnet build' command.
If you still encounter issues, ensure the correct build identity has access to your feed.