[3.13] gh-121103: Put free-threaded libraries in `lib/python3.14t` (GH-121293) (#121631)

On POSIX systems, excluding macOS framework installs, the lib directory
for the free-threaded build now includes a "t" suffix to avoid conflicts
with a co-located default build installation.
(cherry picked from commit e8c91d90ba8fab410a27fad4f709cc73f6ffcbf4)

Co-authored-by: Sam Gross <colesbury@gmail.com>