Sort migrations when embedding them using embed_migrations macro

[silverpill]

By default, WalkDir::into_iter returns entries in the unspecified order:

The order is unspecified but if sort_by is given, directory entries are sorted according to this function.

This code is executed at compile time, so the result is not deterministic and that leads to non-reproducible builds (https://reproducible-builds.org/).

This patch enables sorting of directory entries, which makes the result deterministic.