twister: Look at SIMICS_PROJECT environment variable to find simics

While cmake/emu/simics.cmake uses SIMICS_PROJECT env var, twister checks
if a build is "runnable" by checking if `simics` is executable, but it
doesn't look at SIMICS_PROJECT env var.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>