This disables the update-dependencies cron job in forks. It's not
uncommon for people to leave GitHub Actions enabled in a fork (which in
my experience seems to be the default?), and this unfortunately means
that this job will run in all those forks which is probably not what
people want.
This adds a job to automatically update cargo dependencies once a month.
I've added this script instead of using Renovate because I couldn't get
Renovate to update versions in `Cargo.toml`. I also wanted to batch
transitive dependency updates all in one PR.