util: Updated compiler-test.sh for Python3

In our Ubuntu 18.04 Docker Images, we require gem5 to be build using
`/usr/bin/env python3 /usr/bin/scons ...`.

Change-Id: I4dd3bca1602247575769e6c250337c3ee4a40780
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36884
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
1 file changed