Convert script usage in pipeline YAML to PowerShell Core

As a result of the changes from https://github.com/dotnet/docker-tools/issues/486 and https://github.com/dotnet/core-eng/issues/10097, all of the build agents being used by our builds now have PowerShell Core installed. This means we can take advantage of this commonality by converting script logic in the pipeline YAML to PowerShell Core. This can help to avoid the need for different script logic between platforms. An example of this is this difference between Linux and Windows that required one to be written in Bash and the other in PowerShell:
https://github.com/dotnet/docker-tools/blob/92ef21737f934add62745a505a3d795e88313325/eng/common/templates/steps/test-images-linux-client.yml#L7
https://github.com/dotnet/docker-tools/blob/92ef21737f934add62745a505a3d795e88313325/eng/common/templates/steps/test-images-windows-client.yml#L10

In this instance, we can have a single implementation written in PowerShell that is used by both platforms.