- fetchbzr and nix-prefetch-bzr now only export, instead of cloning

- The option for cloning in nix-prefetch-bzr is removed
- ssl certificates are now ignored by fetchbzr

This means that no .bzr directory is downloaded.  Without this change, the
hash of the result is unpredictable, probably because of timestamping in the
.bzr directory.

Currently, the only package using fetchbzr is kicad.