Update NuGet to version 5.8.1

Fixes: #2027

Fixes the following exception when using OmniSharp on Linux (with Emacs,
but possibly breaks with other LSP clients as well):

    ERROR: OmniSharp.MSBuild.ProjectLoader, The "ResolvePackageAssets" task failed unexpectedly.
    System.MissingMethodException: Method not found: NuGet.ProjectModel.LockFileTarget NuGet.ProjectModel.LockFile.GetTarget(string,string)
      at Microsoft.NET.Build.Tasks.ResolvePackageAssets+CacheReader.CreateReaderFromDisk (Microsoft.NET.Build.Tasks.ResolvePackageAssets task, System.Byte[] settingsHash)
      at Microsoft.NET.Build.Tasks.ResolvePackageAssets+CacheReader..ctor (Microsoft.NET.Build.Tasks.ResolvePackageAssets task)
      at Microsoft.NET.Build.Tasks.ResolvePackageAssets.ReadItemGroups ()
      at Microsoft.NET.Build.Tasks.ResolvePackageAssets.ExecuteCore ()
      at Microsoft.NET.Build.Tasks.TaskBase.Execute ()
      at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute ()
      at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask (Microsoft.Build.BackEnd.ITaskExecutionHost taskExecutionHost, Microsoft.Build.BackEnd.Logging.TaskLoggingContext taskLoggingContext, Microsoft.Build.BackEnd.TaskHost taskHost, Microsoft.Build.BackEnd.ItemBucket bucket, Microsoft.Build.BackEnd.TaskExecutionMode howToExecuteTask)

Currently used NuGet 5.2.0 is too old, update to 5.8.0 makes the issue
disappear.

Author: grendello

build/Packages.props

@@ -5,7 +5,7 @@
         <AspNetCorePackageVersion>2.1.1</AspNetCorePackageVersion>
         <MicrosoftExtensionPackageVersion>3.1.12</MicrosoftExtensionPackageVersion>
         <MSBuildPackageVersion>16.9.0</MSBuildPackageVersion>
-        <NuGetPackageVersion>5.2.0</NuGetPackageVersion>
+        <NuGetPackageVersion>5.8.1</NuGetPackageVersion>
         <RoslynPackageVersion>3.10.0-1.21125.6</RoslynPackageVersion>
         <XunitPackageVersion>2.4.1</XunitPackageVersion>
     </PropertyGroup>