Errors loading GenerateAzureDevOpsBuildManifest in linux

• This issue is blocking
  This is blocking Arcade from flowing to other repos
• This issue is causing unreasonable pain

Arcade -> Arcade-Validation Dependency flow PR dotnet/arcade-validation#1409 is failing in linux legs with:

/__w/1/s/.packages/microsoft.dotnet.arcade.sdk/5.0.0-beta.20213.5/tools/Publish.proj(250,5): error MSB4062: The "Microsoft.DotNet.AzureDevOps.Build.GenerateAzureDevOpsBuildManifest" task could not be loaded from the assembly /__w/1/s/.packages/Microsoft.DotNet.AzureDevOps.Build/5.0.0-beta.20213.5/tools/netcoreapp2.1/Microsoft.DotNet.AzureDevOps.Build.dll. Could not load file or assembly '/__w/1/s/.packages/Microsoft.DotNet.AzureDevOps.Build/5.0.0-beta.20213.5/tools/netcoreapp2.1/Microsoft.DotNet.AzureDevOps.Build.dll'. The system cannot find the file specified.
/__w/1/s/.packages/microsoft.dotnet.arcade.sdk/5.0.0-beta.20213.5/tools/Publish.proj(250,5): error MSB4062:  Confirm that the <UsingTask> declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask.

This was introduced in #5210

CC @chcosta

[chcosta]

ack, looking

[chcosta]

Unfortunately, this succeeds on my local repro attempts when building against the same merge branch and running in the same docker container. I'm continuing to investigate.

[chcosta]

I'm stumped, can't get a repro, but this clearly repro's consistently in CI. I'll continue to investigate tomorrow morning.

[chcosta]

I'll prepare a PR to change the manifest generation to opt-in while I investigate.