SCons: Process platform-specific flags earlier

Some of the logic in SCons depends on flags that get overridden in the
platform-specific `detect.py`, so it needs to be processed first.

For example the Android/iOS/Web platforms override the default `target`
to `template_debug`, but this was processed too late so e.g. the logic
that sets `env.editor_build` would set it to true due to the default
`target` value in the environment being `editor`.

platform/ios/detect.py

@@ -48,7 +48,7 @@ def get_doc_path():
 
 def get_flags():
     return {
-        "arch": "arm64",  # Default for convenience.
+        "arch": "arm64",
         "target": "template_debug",
         "use_volk": False,
         "supported": ["mono"],