build: run official builds with the R1 network isolation policy (#18753)

This required removing connections during the build to `nuget.org` and
`powershellgallery.com`.

The NuGet Tool task was downloading nuget from `nuget.org`
unconditionally.

The `AzureFileCopy` task was downloading `Az.Accounts` from
`powershellgallery.com` unconditionally.

Both of these tasks have better options nowadays.

Tested and passed in OneBranch on 2025-04-01.