Make generated SDK files readonly

[rkr35]

Currently the generated SDK files are not readonly.

If the programmer accidentally performs a write in one of these files—because they, for example, had FSD.rs open in their editor and saved the file with an unintentional change—then rustc compiles the entire SDK crate again. This recompilation can take a minute or longer, which is annoying compared to the seconds of an incremental compile of a clean SDK.

To prevent these accidental SDK recompiles, we should set the readonly flag on the generated files.