scons: Fix the scons script failing to install git hooks

scons: Fix the scons script failing to install git hooks

A recent change,
https://gem5-review.googlesource.com/c/public/gem5/+/38616,
removed the 'root' environment variable, while the git hooks
installation script relies on the variable. This commit
addresses this problem.

Change-Id: I2d27861794021a6a60ef8668461e4127cb1db261
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38655
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>