scons: allow building gem5 in a nix environment

Binaries in nixpkgs are wrapped to use NIX_ environment variables, so
add these to the variables used by scons so that gem5 can be built on a
system using nix.

Change-Id: Iad59db6a4c84d8c7131f309af83618ced1624cfb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50257
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/gem5_scons/defaults.py

@@ -55,6 +55,7 @@ def EnvDefaults(env):
         "DISTCC_",         # distcc (distributed compiler wrapper) config
         "INCLUDE_SERVER_", # distcc pump server settings
         "M5",              # M5 configuration (e.g., path to kernels)
+        "NIX_",            # wrapped binaries if using nix package manager
         ]
 
     for key,val in sorted(os.environ.items()):