SCons: Use default env["ENV"] and prepend PATH to it
SCons: Use default env["ENV"] and prepend PATH to it

This fixes a regression from #46774 where `env["ENV"]` would miss some
important env variables on Windows, such as `SystemRoot`, `PATHEXT`, etc.

So we go back to the previous setup (letting SCons initialize `env["ENV"]`
as it sees fit for the host OS) but use `PrependENVPath` instead of
`AppendENVPath` to preserve the intended fix from #46774.

Fixes #46790 for 3.2.

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot