Create nightly job to bump clippy rust version

[notmandatory]

We should do what the rust-bitcoin team does to have CI check weekly if the latest nightly rust version can run clippy without errors. If clippy passes then CI should open a PR to bump the rust version for clippy. If not we can create an issue to fix later.

"In rust-bitcoin we have started pinning our nightly compiler (which is where we run clippy) and we have a cronjob that runs every so often (was daily, but we reduced it to twice a week, and realistically it'd be fine if it were twice a year) which opens a PR to bump the pin.

If the PR passes CI we know that we can accept it and increase our pin. If it fails CI then we can see that there's an upcoming lint failure and we can address it at our own pace."

Originally posted by @apoelstra in #1430 (comment)