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.

platform/linuxbsd/detect.py

@@ -12,7 +12,6 @@ def get_name():
 
 
 def can_build():
-
     if os.name != "posix" or sys.platform == "darwin":
         return False
 
@@ -81,12 +80,10 @@ def get_opts():
 
 
 def get_flags():
-
     return []
 
 
 def configure(env):
-
     ## Build type
 
     if env["target"] == "release":