Update arcade from build 20200505.6

[safern]

We need to update arcade to unblock official builds due to: dotnet/arcade#5417 and also we need: dotnet/arcade#5415 in order to be able to update TFM from netcoreapp5.0 to net5.0.

Official build test: https://dev.azure.com/dnceng/internal/_build/results?buildId=632553&view=results

cc: @dotnet/runtime-infrastructure

[ghost]

Tagging subscribers to this area: @ViktorHofer
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[safern]

cc: @riarenas @mmitche

[safern]

@ViktorHofer I needed to rename the package testing settings dir to net5.0 to match the new TFM in package testing. We can drop this change and not update that package instead and update it in your PR if you want as well.

Also, it seems like updating arcade is blocked for installer builds that try to build the tests even when not wanting to run them using SkipTests=true property.

dotnet/arcade#5347 (comment)

We can stop building the tests for those cases instead on the meantime to consume arcade.

[ghost]

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