ci: do not export BSIM env vars if bsim is not available

When running run_ci.sh locally with:

 ./scripts/ci/run_ci.sh -l -b master -R upstream/master..

we export BSIM_OUT_PATH unconditionally which causes sanitycheck to fail
on nrf52_bsim (it depends on BSIM_OUT_PATH variable).

Check if the path defined in BSIM_OUT_PATH is available and unset env.
variable if it is not.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>