MSVC: Future-proof installation file skip logic.

MSVC: Future-proof installation file skip logic.

This code relied on knowing exactly where in the source tree temporary
installations might appear.  A reasonable hacker may not think to update
this code when adding use of a temporary installation, making it
fragile.  Observe that commit 9fa8b0ee90c44c0f97d16bf65e94322988c94864
broke it unnoticed, and commit dcae5faccab64776376d354decda0017c648bb53
fixed it unnoticed.  Back-patch to 9.5 only; use of temporary
installations is unlikely to change in released versions.