Auto-update toolchain pin

[sethp]

It seems there was a moment when clippy forgot how to parse arguments ( rust-lang/rust-clippy#9079 ), which unfortunately happens to be the version I had locally.

So instead of using an implicitly moving tag ("nightly"), we've got an explicit nightly that we're using as of de714be. This is much better for reproducibility and auditability, but calls for a robot's help in keeping that pin up to date.