SCons: List .gen.cpp sources explicitly to avoid globbing errors

Whenever we change the name (or remove) generated cpp files with the `.gen.cpp`
extension, users run into build issues when switching between branches (i.e.
switching before and after the name change/removal). This is because we glob
`*.cpp` so if a now-obsolete file from a previous build is present, we'll
include it too, potentially leading to bugs or compilation failure (due to
missing headers or invalid code).

So globbing patterns in `add_source_files` will now skip files ending with
`.gen.cpp`, which should instead be passed explicitly where they're used.

(cherry picked from commit c133480531)

core/SCsub

@@ -151,6 +151,7 @@ env.core_sources += thirdparty_obj
 # Godot source files
 
 env.add_source_files(env.core_sources, "*.cpp")
+env.add_source_files(env.core_sources, "script_encryption_key.gen.cpp")
 
 # Certificates
 env.Depends(