Update to newer rust-mos: integer width

[mlund]

In the mrkits/rust-mos:13f2838f9-334fc98-8f3a80f8 docker image ffi:c_int is 32-bit whereas it should be 16-bit as in earlier versions (see https://github.com/mrk-its/rust-mos/issues/35). This affects the mega65-libc bindings which prevents us from updating.