SCons and python3 issues

[xyzsam]

gem5 has not yet fully migrated to Python3, but the version of Scons we use in the docker image has, which causes issues like this if the user just runs scons build/X86/gem5.opt:

TypeError: a bytes-like object is required, not 'str':
  File "/workspace/gem5-aladdin/SConstruct", line 393:
    main['GCC'] = CXX_version and (CXX_version.find('g++') >= 0 or \

For now, a workaround is to use the command: python2.7 `which scons` build/X86/gem5.opt.