util: Install scons 3.1 from pip in gcn-gpu dockerfile

util: Install scons 3.1 from pip in gcn-gpu dockerfile

A previous commit updated the minimum required version of scons to 3.0

The gcn Dockerfile previously installed scons from apt, which installed
scons 2.4, as the Dockerfile is based on Ubuntu 16

This patch installs scons through pip, which installs scons 3.1

Change-Id: I4f731b301f97e25c730df26afde20ae1cdfaa1b3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34075
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Daniel Gerzhoy <daniel.gerzhoy@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>