Validate isolated stable feed creation functionality

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

We currently don't have any functional validation in place for the pieces of the publishing code that creates isolated stable feeds, which is how https://github.com/dotnet/core-eng/issues/10619 got unnoticed for so long. This section of the code only runs when we're about to RTM, and for servicing releases afterwards.

I think we can do some work to validate this functonality in arcade-validation:

* Set a build leg to generate stable packages and publish them. That should generate an isolated feed every run and publish to it.
* Delete the feed once the build is successful (Arcade validation's packages are never released, so we would just end up creating non-empty feeds over and over.