Install .NET 5.0.103.

Context: https://discord.com/channels/732297728826277939/732297952676020316/819978878948868166
Context: microsoft/vstest#2571

Looks like an updated version of `dotnet test` *might* fix my
"test architecture" conundrum, so let's try using .NET 5.0.103!

Author: jonpryor

build-tools/automation/azure-pipelines.yaml

@@ -18,7 +18,7 @@ pr:
 variables:
   RunningOnCI: true
   Build.Configuration: Release
-  DotNetCoreVersion: 3.1.300
+  DotNetCoreVersion: 5.0.103
   HostedMacImage: macOS-10.15
   HostedWinVS2019: Hosted Windows 2019 with VS2019
   NetCoreTargetFrameworkPathSuffix: -netcoreapp3.1