scons does not use os environment by default:
  https://scons.org/doc/2.1.0/HTML/scons-user/x1750.html

nixpkgs' cc-wrapper on the other hand relies on various NIX_* variables
to be passed through like NIX_CFLAGS_COMPILE_BEFORE.
--- a/SConstruct
+++ b/SConstruct
@@ -559,10 +559,7 @@ options = dict(
     SHLINKCOMSTR=link_shared_library_message,
     LINKCOMSTR=link_program_message,
     PREFIX=GetOption('prefix'),
-    ENV = dict([ (key, os.environ[key])
-                 for key in os.environ
-                 if key in ['PATH', 'TERM', 'HOME', 'PKG_CONFIG_PATH']
-              ])
+    ENV = os.environ,
 )
 
 if ARGUMENTS.get('VERBOSE') == "1":