scons: Pass the CPATH environment variable through to SCons.

For sandbox environment, the default include may be override by CPATH.
To make the SCons can work in this environment, we need to pass CPATH
into SCons.

Change-Id: I1015f20a553a2e18595c8d2a89b209ca665879fd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/72038
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

site_scons/gem5_scons/defaults.py

@@ -52,6 +52,7 @@ def EnvDefaults(env):
             "CC",
             "CXX",
             "HOME",
+            "CPATH",
             "LD_LIBRARY_PATH",
             "LIBRARY_PATH",
             "PATH",