[LGTM] C# 503 Service Unavailable Nuget Download

@microsoft-lgtm For the past few days, our C# ASP .NET 3.1 LGTM builds have been failing to download a particular Nuget package:

[2020-12-18 20:14:06] [build]   ServiceUnavailable https://api.nuget.org/v3-flatcontainer/system.runtime.compilerservices.unsafe/4.5.2/system.runtime.compilerservices.unsafe.4.5.2.nupkg 234ms
[2020-12-18 20:14:06] [build] Failed to download package 'System.Runtime.CompilerServices.Unsafe.4.5.2' from 'https://api.nuget.org/v3-flatcontainer/system.runtime.compilerservices.unsafe/4.5.2/system.runtime.compilerservices.unsafe.4.5.2.nupkg'.

Full example: https://lgtm.com/projects/g/sillsdev/TheCombine/logs/rev/pr-0d9f66b83738012dda7a99768876dd6a5dbc7044/lang:csharp

But when we build this locally on our development machines inside or outside Docker, the build works fine. Even simply downloading the file directly using a web browser works. LGTM builds worked a few days ago: https://lgtm.com/projects/g/sillsdev/TheCombine/rev/pr-7dc5b04c53cf22c87ced36da088bfffe78665ae1

Was there a recent change in the default LGTM C# build infrastructure lately that could have affected this?