SCons: Expand env variables to check compiler version

Scons' `Environment.subst()` does that, and was already used in the
other place where we query an env variable (`env["LINK"]` in x11 code).

Fixes `3.2` iOS build after cherry-pick of #36559 (previously it only
ran for GCC code, not iOS's Clang), and the same issue would likely
affect `master` if iOS builds were enabled right now.

platform/x11/detect.py

@@ -1,7 +1,7 @@
 import os
 import platform
 import sys
-from methods import get_compiler_version, using_gcc, using_clang
+from methods import using_gcc, using_clang
 
 
 def is_active():