scons: Pass the DISPLAY environment variable through to SCons.

This lets gui programs run correctly within SCons, specifically the
kconfig "guiconfig" helper utility.

Change-Id: Iec51df3db89ac7e7411e6c08fe8201afb69dc63e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56952
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matthew Poremba <matthew.poremba@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/gem5_scons/defaults.py

@@ -67,6 +67,7 @@ def EnvDefaults(env):
             "LINKFLAGS_EXTRA",
             "LANG",
             "LC_CTYPE",
+            "DISPLAY",
         ]
     )