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>

util/compiler-tests.sh

@@ -116,7 +116,8 @@ for compiler in ${images[@]}; do
             # Build with container
             {
                 docker run --rm -v "${gem5_root}":"/gem5" -u $UID:$GID \
-                    -w /gem5 $repo_name scons "${build_out}" "${build_args}"
+                    -w /gem5 $repo_name /usr/bin/env python3 /usr/bin/scons \
+                    "${build_out}" "${build_args}"
             }>"${build_stdout}" 2>"${build_stderr}"
             result=$?