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/server/detect.py

@@ -21,7 +21,6 @@ def get_program_suffix():
 
 
 def can_build():
-
     if os.name != "posix":
         return False
 
@@ -45,7 +44,6 @@ def get_opts():
 
 
 def get_flags():
-
     return []