scons: Clone the gem5py_env environment in src/SConscript.

This will avoid cross contamination between variants, where the gem5py
executable from one variant is used by another variant, potentially
crashing SCons on a clean build.

Change-Id: I6c1741d431892ff11c2e05a6beb5e87c2b0d67eb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51087
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -80,6 +80,7 @@ build_tools = Dir('#build_tools')
 # Build a small helper that runs Python code using the same version of Python
 # as gem5. This is in an unorthodox location to avoid building it for every
 # variant.
+gem5py_env = gem5py_env.Clone()
 gem5py = gem5py_env.Program('gem5py', 'python/gem5py.cc')[0]
 gem5py_env['GEM5PY'] = gem5py
 # Inject build_tools into PYTHONPATH for when we run gem5py.