[libc++] Require installing dependencies before running LNT tooling (#214020)

This makes all benchmark-related utilities consistently rely on a
virtual environment containing libcxx/utils/requirements.txt instead of
some scripts installing their dependencies explicitly, which is
duplicate work in most cases.

GitOrigin-RevId: 8a8fbc4c8a9f79dba1cbc204d357ae7f8ae1d30e