SCons: Add only selected platform's opts to env

Otherwise we can get situations where platform-specific opts with the same name
can override each other depending on the order at which platforms are parsed,
as was the case with `use_static_cpp` in Linux/Windows.

Fixes #44304.

This also has the added benefit that the `scons --help` output will now only
include the options which are relevant for the selected (or detected) platform.

(cherry picked from commit 0f84d8dc49)

platform/uwp/detect.py

@@ -30,7 +30,6 @@ def get_opts():
 
 
 def get_flags():
-
     return [
         ("tools", False),
         ("xaudio2", True),
@@ -39,7 +38,6 @@ def get_flags():
 
 
 def configure(env):
-
     env.msvc = True
 
     if env["bits"] != "default":