Fix let_it_be usage in nuget_packages_spec.rb

The following discussion from !24182 (merged) should be addressed:

• @DylanGriffith started a discussion: (+1 comment)

  If we're doing extra work to maintain state between test runs I'd prefer to just avoid let_it_be and then just use let so the state is clean each test run. This can lead to really confusing results and hard to debug tests or possibly even false positives.