scons: Build error introduced by site_tools/default.py

scons: Build error introduced by site_tools/default.py

One of the latest commits affecting the build system introduced a change
in the environment setting: rather than using the ENV dictionary,
environment key-val pairs were saved directly into the class. This
patch reverts that behaviour.

Change-Id: If206ba0a34ac8c35695a493cb90654b7ec81971b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5941
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>