Automate bumping pinned toolchain

[0xaatif]

          > Could do a weekly cron-ed PR

If we want to pin the toolchain (which may very well needed given latest mdbook issues and possibly unforeseen incoming ones), then I'd tend to opt for some cron-ed job to keep it relatively up-to-date, at least while we're heavily developing, but this seems tedious if we cannot automate the actual version bump.

Originally posted by @Nashtare in #660 (comment)

Follows #665