Remove feedSource parameter from MicroBuild task

[benvillalobos]

In a conversation between Rainer and Rick Krause

Rick mentions:

You’re using the wrong package feed— https://devdiv-test.pkgs.visualstudio.com/DefaultCollection/_packaging/MicroBuildToolset/nuget/v3/index.json.
Change the feed source in your task to https://devdiv.pkgs.visualstudio.com/DefaultCollection/_packaging/MicroBuildToolset/nuget/v3/index.json

And:

Actually, you could drop the feedSource value altogether, since it defaults to the value I said to use.
Thanks, Rick

It gets passed the step, but fails now at signing validation

[error].packages\microsoft.dotnet.arcade.sdk\1.0.0-beta.20509.7\tools\SdkTasks\SigningValidation.proj(56,5): error : Signing validation failed. Check signcheck.errors.log for more information.

Pipeline Build: https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=4237997&view=results

[Forgind]

Signing problem was https://github.com/dotnet/core-eng/issues/11458, right?

[benvillalobos]

I believe per https://github.com/dotnet/core-eng/issues/11458#issuecomment-730710439 we'd need to add
1.0.0-beta.20569.8
to our eng/versions.props file. This would unblock us on the MicroBuild feedSource issue and the arcade signing issue. PR's would likely have to rebase off of master to pull in the workaround.

This PR + #5893 should fix our pipelines