Merging the nightly update PRs is boring and annoying. We can just do it
once a week without too much risk of falling behind.
This does mean that I (Tobin) can't be lazy and skip merging the bot's
PRs (which I find myself doing when its twice a week).
Currently we run the job at midnight here and in `sepc256k1`, this led
recently to one using the nightly toolchain from the 10th of Sep and the
other using the toolchain from 11th of Sep.
Update to run at 5 past so this doesn't happen again.
Tobin C. Harding
dependabot[bot]