run_plfs_regression.sh no longer set env variables with PLFS build flags

Scripts should now get the build flags directly from
tests/utils/rs_plfs_buildflags_get.py instead of relying on env
variables that run_plfs_regression.sh sets. All tests that needed the
build flags had already migrated to using the script. Only
openmpi_build.sh had to be updated to call the script.