This bump will includes a couple of commits which improve the
reproducibility of the mingw-w64 toolchain. Most of which came from
debian. They will be upstreamed as upstream Guix release timeline
allows.
- Clearer and more accurate prose
- Pin `guix pull' to commit rather than branch
- Just use `use-module' instead of `define-module'
- Use `bash-minimal' instead of `bash'
- Remove unneeded `tcsh' from manifest
- Explicitly use `python-3.7'
- Add comments about how {native,cross}-toolchains are produced and
why