swig: Use SWIG from environment when determining version
swig: Use SWIG from environment when determining version

This patch fixes a minor issue in the SConstruct where a hardcoded
swig is used instead of the environment SWIG when determining the
version.

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin