build: conditionally clear vcinstalldir

For scenario where target env is explicitly specified as vs2019, do
not clear VCINSTALLDIR which was being cleared to handle fallback to
vs2017 block when attempting to find a matching available VS.

Fixes: https://github.com/nodejs/node/issues/35856

PR-URL: https://github.com/nodejs/node/pull/36009


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>