VS: Exclude VS 2019 instances when using VS 2017 generator

Filter instances reported by the VS Installer to consider only VS 2017
instances for the "Visual Studio 15 2017" generator.

Fixes: #18721
Status Job ID Name Coverage
  External
passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager