[OpenMP] Use secure_getenv when read OMPT library path (#208652)

An internal review flagged the use of `getenv` when reading the OMPT
tool library path from the environment and passing it to the loader as a
potential vuln.

This patch replaces the use of getenv in question with the use of
secure_getenv on glibc platforms.
On non glibc platforms, no change is implemented.

GitOrigin-RevId: 7fcfcf1c5db1d46b6cc85b80be3a24ee887773f9