VS: Choose VS 2017 instance via environment variable

In the `Visual Studio 15 2017` generator, if the `VS150COMNTOOLS`
environment variable points at a specific VS 2017 instance reported by
the Visual Studio Installer tool, use that as the preferred instance.

Inspired-by: Iyyappa Murugandi <iyyappam@microsoft.com>
Fixes: #16846
16 jobs for vs-2017-choose-via-environment
Status Name Job ID Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager